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doi:10.2204/iodp.pr.317.2010

Synopsis

The fundamental objective of Expedition 317 was to test the concepts of sequence stratigraphy by documenting the lithologies, ages, and depositional environments within sequences in order to determine the sedimentary processes active during sequence development and constrain the underlying controls (both global and local) responsible for those processes. This involves testing sequence stratigraphic model (e.g., Van Wagoner et al., 1988; Vail et al., 1991) predictions of facies and lithologies associated with the development of prograding, clinoformal, continental margin sequences that are well defined by seismic interpretations. It also necessitates testing the presumption that global sea level is the primary control on sequence boundary formation (Haq et al., 1987). Other goals were coring the early Oligocene Marshall Paraconformity, constraining the early uplift and erosion history of the Southern Alps, and documenting the evolution of contourite deposition on this current-influenced margin.

Nineteen middle Miocene to Pleistocene seismic sequence boundaries (U1–U19) were defined by sequence stratigraphic interpretation of the EW00-01 seismic data (Lu and Fulthorpe, 2004). Expedition 317 was designed to provide ground truth for sequences above boundary U4 and also to core through the Marshall Paraconformity. Prior to Expedition 317, only ODP Leg 174A (Austin, Christie-Blick, Malone, et al., 1998) and IODP Expedition 313 (Expedition 313 Scientists, 2010) had drilled on a continental shelf for sequence stratigraphic and sea level objectives. Expedition 317, therefore, provided a rare opportunity to investigate the facies, paleoenvironments, and depositional processes associated with the sequence stratigraphic model on a prograding continental margin where large-scale depositional geometries and sequence architectures are constrained by seismic data.

Core-seismic correlation

The Canterbury Basin formed along a passive continental margin where large volumes of sediment were derived from a tectonically and climatically evolving hinterland. Seismic sequence boundaries provide a large-scale understanding of the subsurface architecture of the margin necessary for drill site selection and for placing drilling results within a broader two-dimensional and three-dimensional context. Sequence boundaries U19–U6 were penetrated at outer shelf Site U1351 and mid-shelf Site U1353, which may also have reached U5. Intermediate shelf Site U1354 cored U19–U8. Slope Site U1352 penetrated slope facies of most seismic sequence boundaries (U19–U13, U11, and U9–U6), as well as the Marshall Paraconformity, with the particular goal of providing age control for sequence boundaries. Slope facies of sequence boundaries U6–U7 were also sampled at shelf Site U1351 (Figs. F13, F14).

Upper Miocene–lower Pliocene seismic sequence boundaries (~U10 and below) feature smooth, onlapped paleoshelves and rounded rollovers, or clinoform breaks, with sigmoid internal reflection geometries. In contrast, mid-Pliocene to Pleistocene sequence boundaries (above U10) display eroded and incised downlapped paleoshelves and more pronounced rollovers with oblique reflection geometries. This transition reflects a background of changing paleoenvironmental conditions, against which the cored sediments can be interpreted downhole.

Similarly, the character of sequences changes horizontally landward to basinward across the transect. Sequences are not represented by a single pattern of lithofacies across the shelf and onto the slope. Shelf successions are dominated by periodic facies changes that reflect fluctuating sea level, whereas deposition of slope sediments was also influenced by north-traveling Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and their predecessor current systems (e.g., Carter and Gammon, 2004).

Correlating seismic sequence boundaries with lithologic facies and features found in the cores will be a focus of postcruise research. However, preliminary relationships can be defined at two scales.

Lithologic units

At the largest scale, the preferred boundaries between lithologic Units I and II at both outer shelf Site U1351 and inboard shelf Site U1353 are lithologic boundaries that separate overlying heterolithic facies of Unit I from more uniform facies below in Unit II (Figs. F13, F14, F15). The unit definitions for each of the four Sites U1351–U1354 are based on the observed variation in lithology in the cores, particularly in the occurrence and frequency of calcareous units (generally green, shelly, and sandy). In general, Unit I is heterogeneous, containing a wide variety of facies including interbedded terrigenous lithologies and many green marl or calcareous beds with sharp (or bioturbated) bases (Fig. F15). Conversely, Unit II is more homogeneous. On the shelf, Unit II is dominated by mud or muddy sand, with lower percentages of carbonate components and less frequent greenish calcareous beds, whereas on the slope this unit is represented by homogeneous sandy marlstone (Figs. F15, F16). However, at each site the location of the unit boundary is difficult to identify precisely because the transition between units of this type is inevitably gradational and also because of low recovery near some unit boundaries (Fig. F17). At slope Site U1352, the homogenous sandy marlstone of Unit II changes abruptly into foraminifer-bearing nannofossil limestone of early Oligocene to late Eocene age. This hemipelagic to pelagic unit is correlative with the onshore Amuri Limestone.

The Unit I/II boundary generally conforms to the predicted depths of seismic sequence boundaries U10 and U12 (late Pliocene) in a zone between high-amplitude seismic reflections related to clinoforms with sharp shelf-slope transitions above and clinoforms with rounded shelf-slope transitions below. The exact location is not identical at each site, in part because of the gradational nature of the transition noted above.

One objective of drilling on the shelf was to test the hypothesis that these contrasting characteristics of sequence boundaries and seismic facies above and below ~U10 occur because the paleoshelves of U10 and the underlying sequence boundaries were not subaerially exposed at lowstand, whereas those above U10 may have been exposed, probably because of increasing eustatic amplitudes during the Pleistocene. The lithologic contrasts between Units I and II at shelf Sites U1351 and U1353 appear to broadly support this hypothesis, with the heterolithic sediments of Unit I representing deposition under conditions of fluctuating sea level since the mid-Pliocene. The fact that the transition is gradational also implies that the change in processes operating to produce these sediments is also gradational and not an abrupt change from one type of process that produces rounded shelf-slope breaks to one that produces sharp shelf-slope breaks.

The change in lithology between units is mirrored in the percentage of carbonate components as estimated from smear slides and thin sections and measured using X-ray diffraction (XRD) and coulometry. The carbonate percentage in Unit I is high and variable (Fig. F16). The carbonate percentage in Unit II at shelf sites is low and only variable at the top of the unit, whereas it is moderate and less variable at slope Site U1352 (Fig. F16). At shelf Site U1351, the carbonate percentage begins to increase downhole, which is a reflection of the shift of depositional environment toward the slope with depth at this site. At slope Site U1352, the carbonate percentage also increases downhole, reflecting a rise in oceanicity (planktonic foraminiferal abundance) of the depositional environment (Fig. F16). Subunit IIC and Unit III are both limestones, deposited in truly oceanic environments, with little to no terrigenous sediment input.

Potential sequence boundaries in cores

Shelf sites (U1351, U1353, and U1354)

Finer scale correlations between lithology and seismic interpretations are also possible. Correlation of specific lithologic features with seismic sequence boundaries will be improved by postcruise analysis, but some intriguing relationships can be drawn based on shipboard observations. Erosion surfaces and associated overlying sediments were identified near the predicted depths of seismic sequence boundaries at all sites (Figs. F18, F19). Identification is based on changes in lithologic composition, the presence of reworked sediments, and the nature of the contacts. Basal erosion surfaces were not always recovered—for example, in zones where recovery was poor because of the (inferred) presence of sand. In such cases, the surfaces were picked based on the presence of sand (or the assumption of washed-out sands) and correlation to wireline logs. Prefixes in the erosion surface numbering sequence link erosion surfaces to the sites at which they were identified (Figs. F18, F19), but these lithologic surfaces and associated sediments are believed to be correlative between sites across the transect.

Candidate sequence boundaries are best identified in lithologic Unit I at each site, where recovery was highest (Fig. F17). For example, the erosion surfaces of Unit I at Site U1351 are overlain by a series of lithofacies associations composed of upward-fining shelly sandy mud topped by regressive coarsening-upward sandy mud. Together, the package is provisionally interpreted as a transgressive wave-eroded ravinement surface overlain by a lag deposit corresponding to a transgressive systems tract. Subsequent highstand mud units are topped by the regressive coarsening-upward sediments. The facies assemblage includes decimeter- to centimeter-thick clay beds similar to those attributed to rapid deposition by flood events on modern shelves near sediment-rich fluvial systems. Truncated versions of this facies association, lacking the regressive highstand phase, occur in the uppermost 30 m at Site U1352, suggesting more frequent erosive episodes. There are more such truncated facies associations than there are seismic sequence boundaries, and it is therefore possible that some of the truncated facies associations may represent higher order cycles or autocyclicity related to migration of distributaries and/or shelf depocenters. The facies associations, both complete and truncated, are similar to those known from the Wanganui Basin, between North Island and South Island of New Zealand (Abbott and Carter, 1999; Saul et al., 1999; Abbott et al., 2005; Naish et al., 2005).

Unit I erosion surfaces at Site U1351 may be correlative with sequence boundaries U19–U11 (Fig. F19). The geometries of seismic sequence boundaries U19–U11, as well as paleoenvironmental interpretations from biostratigraphy, show that these erosion surfaces developed on the mid- to outer shelf, where eustatic fluctuations had the most significant direct impact. In contrast, erosion Surface E8 and the underlying strata formed when the position of Site U1351 was on the outermost shelf, progressively migrating (downhole) to the upper slope. E8 is less sharp than E1–E7, and reworked sediments are more difficult to identify below E8.

No erosion surfaces were identified in Unit II at Site U1351, although cemented intervals occur, some of which may correlate with seismic sequence boundaries. Diagenetic changes begin in the lower part of Unit I (at ~150 m) with authigenic carbonate cements and limestone concretions. Unit II is dark greenish gray very fine sandy mud and greenish black to dark greenish gray mud. Shells are less frequent below ~350 m and are absent below ~890 m, probably reflecting the downhole shift from shelf to slope environments. The indurated clayey or sandy mud found in some core catchers in the lower part of the unit was drilling related, caused by frictional heating and baking by the XCB cutting shoe. However, other indurated intervals represent in situ carbonate cementation. These nodules or concretions are present throughout the unit, particularly below 750 m. These have no clear relationship to intervals of low recovery.

The facies associations noted at Site U1351 may also be present in lithologic Unit I at Site U1353, but they are not as prominent. The uppermost four erosion surfaces identified at Site U1353 correlate well with seismic sequence boundaries U19–U16 (Fig. F19). Correlations of other erosion surfaces and associated sediment packages at Site U1353 with seismic sequence boundaries remain tentative because of poor recovery (Fig. F17). Below 80 m, recovery at Site U1353, the most landward of the shelf sites, was either low or involved significant cave-in of overlying shell hash. The dominant facies is mud, interpreted mostly to represent a highstand facies. Sandy intervals represent transgressive or regressive periods, whereas the shell hash, even when muddy, probably formed as a transgressive shoreface deposit. Sediment recovered from Unit II was predominantly muddy, but recovery was low and other lithofacies may have been present. The mud suggests deposition on a shelf below the fair-weather wave base, and the lack of abundant shell debris suggests that the setting was more distal than it was during deposition of Unit I. The sediment source for terrigenous sediments of both Units I and II at Site U1353 appears to have been the Torlesse graywacke to the west. This contrasts with sediments in Unit II at outer shelf Site U1351 and Subunit IB and Unit II at slope Site U1352, which include a major contribution from the Otago schists to the south.

Site U1354 facies associations show close similarity with those of the other shelf Sites U1351 and U1353. Several potential unconformities are recognized within Unit I. Sequence boundaries U19–U16 were intersected in both Holes U1354A and U1354B at depths comparable to those predicted (Fig. F19). In many cases, more than one candidate surface in the core could correlate with seismically mapped surfaces. Correlations with seismic sequence boundaries therefore remain tentative, but good recovery in the uppermost 250 m at Site U1354 should allow for improved correlation between lithologic features and seismic sequence boundaries U19–U10. As at other sites, poor recovery in the lower part of the hole makes core-seismic correlation more difficult.

Slope Site (U1352)

A number of distinct surfaces and their associated sediments present at slope Site U1352 also have the potential to be resolvable by seismic profiles. Whereas some candidate sequence boundaries involve sharp basal contacts separating different lithologies, as at Site U1351, others fall within intervals containing several coarse-grained packages, so a specific sequence bounding surface may be difficult to pick. Coarse sandy intervals potentially correlate with seismic sequence boundaries U19–U16. These remain the best constrained boundaries. However, because of improved recovery at Site U1352 (Fig. F17), candidate slope erosional surfaces were also identified in all lithologic units down to and including the Marshall Paraconformity (Fig. F18).

Sedimentary cyclicity

Seismic sequences and Milankovitch cyclicity

The relationship between high-frequency Milankovitch sequences and longer, seismically resolvable sequences remains an active and unresolved area of research. Saul et al. (1999) showed that for the Wanganui Basin, classic fourth- and fifth-order cyclothems are grouped together into third-order cycles that were controlled by basinwide tectonic rhythms. Alternatively, Miller et al. (2005) pointed out that some third-order cyclicity may be the manifestation of longer wavelength Milankovitch periodicities of 1.2 and 2.4 m.y. or of interference between these cycles.

Sequences drilled during Expedition 317 span the boundary between Milankovitch-scale cyclicity and longer period, third-order, seismically resolvable cycles: periods of most seismic sequences range from ~0.1 (seafloor to ~U17) to ~0.4 m.y. (U16–U7) (Table T2; Fig. F20). The two upper seismically delimited sequences on the Canterbury shelf (corresponding to the sequences overlying seismic sequence boundaries U18 and U19) almost certainly correspond to marine isotope Stage (MIS) 1–5 and MIS 6–7 sedimentary cycles based on the magnetic susceptibility (MS) and natural gamma ray (NGR) time series (Fig. F21). Erosion Surfaces E2–E4 at Site U1351 correlate in turn with seismic sequence boundaries U18–U16, respectively. Gamma ray trends between successive low gamma spikes in this interval show four cycles of a gradual upward increase followed by a decrease. These cycles correlate with fining- to coarsening-upward facies associations, which are formed by one cycle of transgression and regression (e.g., E2–E4 at Site U1352; Fig. F18). Based on biostratigraphic datums and the inferred position of the Brunhes/Matuyama boundary (~70 m at Site U1351), we can speculate that these sequence boundaries formed during periods of low global sea level and perhaps correlate with three of the four MIS 8, 10, 12, and 16 (Fig. F22). At Site U1354, a burrowed probable sequence boundary in the same interval that contains the Brunhes/Matuyama boundary corresponds to seismic sequence boundary U16. A ~0.3 m.y. gap that encompasses the Jaramillo normal chron, demonstrated on nannofossil evidence, corresponds to sequence boundary U15. Furthermore, other, older seismic sequence boundaries correspond to erosional gaps in the succession. Three distinct intervals of low gamma radiation and high resistivity in wireline logging data across the three shelf sites correspond well to the predicted depths of sequence boundaries U12, U11, and U10 (Fig. F23), which are also well resolved in preliminary synthetic seismograms generated from the logs. These features, which most frequently coincide with intervals of poor core recovery, appear to be associated with several meter thick sand, shell, and gravel beds that may be laterally continuous across the shelf.

Postcruise analysis, including improved age control for deeper sequence boundaries, will document and clarify the probable relationship between seismic sequences and Milankovitch cycles. Careful study will be needed to distinguish lithologic changes that correspond to genuine Milankovitch cyclicity from those that result from autocyclic variation within cycles, controlled by sedimentary changes of local significance. It is already clear that Expedition 317 data will provide important insights into the hierarchy of sedimentary cyclicity and into how and why certain cycles and/or periodicities are preferentially imaged using seismic data.

Late Quaternary to Holocene glacial–interglacial cycles

Expedition 317 shelf sites (landward to basinward: U1353, U1354, and U1351) provide highly significant high-recovery sections through the Holocene and late Quaternary for high-resolution study of recent glacial cycles in a continental shelf setting. The smaller scale cycles penetrated by Expedition 317 drilling are in part similar to the Milankovitch-scale rhythms documented from nearby ODP Site 1119 (Shipboard Scientific Party, 1999a; Carter and Gammon, 2006) and also the New Jersey shelf (McHugh and Olson, 2002) and correspond to cycles with inferred lengths of 100,000 and 40,000 years. The seismic stratigraphy of the sequences deposited during the last (MIS 1–5) and penultimate (MIS 6–7) glacial–interglacial cycles on the Canterbury and Otago shelves has been described by Browne and Naish (2003) and Osterberg (2006).

Cycles drilled during Expedition 317 have differing sedimentary architectures dependant upon whether they were deposited seaward (Site U1352) or landward (Sites U1353 and U1354) of the inferred position of the last glacial lowstand shoreline (~125 m). Site U1351 at 122 m water depth is located near the lowstand shoreline. Cycles at slope Site U1352 were always seaward of the lowstand shoreline. The main sedimentary facies association on the slope is composed of a sharp basal contact with sand-filled Thalassinoides burrows overlain by a few decimeters of shelly sand that grades upward to sandy mud. Similar cycles at nearby Site 1119 were shown to contain temperate-water planktonic foraminifers in the basal sand (Shipboard Scientific Party, 1999a) with a colder water microfauna in the intervening muds; these facts are consistent with each such cycle representing a full interglacial–glacial rhythm (though perhaps with a diastem of a few thousand years represented by the sharp, burrowed base to the sand units). These rhythms are interpreted as resulting from oscillations in flow intensity of cold-water SAMW, perhaps linked also to migrations of the Southland (Subtropical) Front across the site (Carter et al., 2004b). The rhythms also have to be influenced by terrestrial sediment supply, which is likely highest at lowstands and minimal to absent during highstands.

Glacial–interglacial deposits landward of the position of the lowstand shoreline are well developed in the upper parts of Sites U1353 and U1354. They too are composed of a sharp and sometimes burrow-penetrated base, inferred to sometimes represent a shoreface ravinement surface, overlain by well-sorted sands up to several meters thick that grade rapidly up into sandy siltstone and siltstone. A pedogenic profile may be preserved below one or more of the ravinement surfaces that coincide with preexisting sequence boundaries; if confirmed, this would be an unusual circumstance because virtually all of the Pleistocene sequence boundaries in the onland Wanganui Basin are characterized by sharp (albeit often burrowed) marine ravinement surfaces cut during transgression, and only one soil profile has been recorded (Abbott, 1992).

Some basal sandy units have a lower shelly portion or possess a shell-hash facies similar to the Type A transgressive shellbeds described from mid-Pleistocene cyclothems in the onland Wanganui Basin (Abbott and Carter, 1994). The associated molluscan faunas are firmly indicative of a proximal shoreface–beach environment for the sandy and shelly units (Austrodosinia, Phacosoma, Anchomasa, Maoricolpus, and Alcithoe), and a distal shoreface–inner shelf environment (Atrina, Dosinia lambata, Divaricella, Maorimactra, Scalpomactra, Antisolarium, and Stiracolpus) for the slightly deeper water siltstones.

Overall, the sedimentary cycles penetrated in the upper parts of Sites U1353 and U1534 show striking similarities to Pleistocene cyclothems described from the Wanganui Basin. Though in detail individual cycles display differing sedimentary architectures, which await full description and interpretation, elements of the Castlecliff motif (Abbott and Carter, 1999; Saul et al., 1999) and the Birdgrove and Maxwell motifs (Abbott et al., 2005) may all be represented.

Located close to the position of the lowstand shoreline, Site U1351 displays cyclicity characteristic of the more shoreward facies associations in its upper part and of the upper slope facies associations deeper in the hole. However, the uppermost cycles at Site U1351 are thicker and represent deeper water mud facies settings than those of their shelf counterparts at Sites U1353 and U1354, as would be expected. Some Site U1351 cycles coarsen upward into sands before being truncated by the superjacent sequence boundary, in similar fashion to Pliocene cycles described from the Wanganui Basin (Naish et al., 2005).

Sedimentary processes

One goal of Expedition 317 was to provide insights into the role of contour current deposition in a location where prominent sediment drift geometries are absent. Slope Site U1352 is most relevant to this question, but the deeper parts of Site U1351 also sampled slope facies between sequence boundaries U6–U8.

Northeasterly flowing currents (Southland Current, inboard of the Subtropical Front, and a gyre of the Subantarctic Front) are active today. At present, the core of the Southland Current is over the ~300 m isobath (Chiswell, 1996). The local gyre of the Antarctic Circumpolar Current circulates clockwise within the head of Bounty Trough parallel to the Southland Current to at least 900 m (Fig. F2) (Morris et al., 2001). The sediment drifts in the northern part of the Canterbury Bight, as well as outcrop evidence of current-related deposition, suggest that similar currents have been active for the last 30 m.y.

Early Pliocene and late Miocene nannofossil assemblages at Site U1351 contain reworked older material, some as old as Oligocene, but mainly early to middle Miocene. The downhole increase in reworking began while paleowater depths were still relatively shallow (inner to mid-shelf) and before the downhole transition from shelf to upper slope facies, between sequence boundaries U8 and U7 (~725 m), where paleowater depths became more equivalent to those influenced by modern ocean currents. Nannofossils are small and easily transported. Reworked nannofossils at Site U1351 may have been reworked from the Campbell Plateau, where Miocene sediment is shallowly buried and may be exposed. This suggests an along-strike component of deposition.

A nearly complete Neogene continental slope sedimentary record dominated by pelagic to hemipelagic sedimentation with minor traction and gravity flow sediments and, below the Marshall Paraconformity, a condensed late Eocene to early Oligocene section was cored at slope Site U1352. This site represents a downhole record from unlithified sediments to lithified carbonates at depth. The gradual downward transition in lithofacies from more siliciclastic-rich Quaternary muddy facies into pelagic limestones and glauconitic marlstones and marls appears to reflect the downhole transition, seen on seismic profiles, from an upper slope location on a clinoformal margin with sharp shelf-slope breaks in the Quaternary toward a toe-of-slope location on a margin with more rounded shelf-slope breaks and more gently inclined slope in the Miocene (Fig. F14). The lower carbonate content in the upper part of this interval may be linked to higher terrigenous input, related to the uplift of the Southern Alps and/or Neogene climate change.

Subunit IA at Site U1352 may be composed of sediment gravity flows that fed a series of lowstand deltas. Evidence for this lies in the composition of the sediments. Gray sand beds of Subunit IA have a Torlesse terrane mineralogy, indicating that they derive from the Canterbury region. This differs from the rest of Site U1352 (Subunit IB and below) and Site U1351, which appear to be predominantly sourced from the Otago schist to the south. Subunit IB also seems likely to contain evidence of mass transport deposition, although the equivalent interval at ODP Site 1119 was interpreted as contourites. Subunit IB may be a transitional (downhole) downslope to drift interval culminating in Subunit IC, which shows evidence (well-sorted, very fine grained sand fraction) consistent with a considerable transport distance and seismic evidence (strike profile) of sediment drift deposition.

Unit II is dominantly calcareous (sandy marl and marlstone). Hemipelagic to pelagic sediments of Subunit IIA (709–1189 m) may have been deposited as sediment drifts, based primarily on seismic evidence downslope and along strike (Fig. F14): diagnostic sedimentary structures are rare, in part because of extensive bioturbation. More calcareous lithologies may represent condensed intervals within the drifts. The increasing frequency of calcareous intervals toward the base of Subunit IIA may reflect reduced rates of terrigenous deposition near clinoform toes.

Subunit IIB (1189–1694 m) contains intermittent current-generated structures (wavy laminations and ripple cross laminations) interpreted as contourites. An alternation of light-colored marlstone with darker mudstone suggests switching between more hemipelagic carbonate (highstand?) sources and terrigenous (lowstand?) sources. Subunit IIB lithologies suggest a complex interplay between along-strike and downslope sedimentary processes. Subunit IIB also contains mass transport complexes (slumping) near its base.

Subunit IIC marlstones and limestones lack traction features, suggesting pelagic deposition from suspension in deep water. Micropaleontology indicates a lower bathyal setting. The intercalated glauconitic sandstone laminae characteristic of this unit are inferred to be injected sands derived from the underlying Oligocene greensand (Kekenodon Group), which was not recovered at this site. The Marshall Paraconformity (see below) forms the boundary between Units II and III. Unit III is Amuri Formation–equivalent pelagic nannofossil limestone deposited in lower bathyal depths.

Marshall Paraconformity

The Marshall Paraconformity is a regional unconformity terminating the postrift transgressive phase and capping the widespread Amuri limestone formation. The paraconformity has been dated at its onshore type section using strontium isotopes as representing a hiatus of 32.4–29 Ma (Fulthorpe et al., 1996), where it occurs at the base of mid–late Oligocene cross-bedded glauconitic sand (Concord Formation) and calcarenite limestone (Weka Pass Formation). The paraconformity was the deepest target of Expedition 317 drilling and is hypothesized to represent intensified current erosion or nondeposition associated with the initiation of thermohaline circulation (Deep Western Boundary Current) and associated shallower circulation upon the opening of the seaway between Antarctica and Australia (~33.7 Ma), prior to the opening of the Drake Passage (Carter, 1985; Fulthorpe et al., 1996; Carter et al., 2004c).

The Marshall Paraconformity is represented by a lithologic change at 1853 m in Hole U1352B, requiring a record penetration for a Deep Sea Drilling Program (DSDP)/ODP/IODP hole drilled during a single expedition (total depth was 1927 m). The paraconformity marks the boundary between overlying lower Miocene glauconitic limestone and underlying lower Oligocene recrystallized pelagic nannofossil limestone containing trace fossils and stylolites, equivalent to the onshore Amuri Formation. Recovery was low across the paraconformity, which is represented by a decimeter-scale rubble zone in recovered core. No equivalent to the Concord Formation glauconitic sand was recovered, although logs from the Clipper-1 exploration well suggest that a boundary sand layer is present. The hiatus at the paraconformity is estimated to be 11–12 m.y. at Site U1352, conforming with previous observations that the hiatus is longer offshore than at the type section, perhaps owing to greater current-induced erosion at such deeper water locations (Shipboard Scientific Party, 1999a; Carter et al., 2004c). Hole U1352B terminated in upper Eocene (35.2–36.6 Ma) limestone with clayey interbeds.

Drilling results indicate that the Marshall Paraconformity correlates with the reflection named "Green" by industry interpreters (e.g., Hawkes and Mound, 1984). Our original interpretations assumed a correlation with a reflection that is ~160 m shallower than "Green." This, in turn, means that "Green" does not represent the base of the Amuri Formation limestone, as was previously inferred.

Biostratigraphy

Expedition 317 cored subtidal to lower bathyal sediments of Holocene to late Eocene age (0–36 Ma) in a transect across the Canterbury Basin continental shelf and slope. Sequence stratigraphic processes influenced calcareous nannofossil and planktonic foraminiferal assemblages, and this is strongly reflected in the overall abundance, preservation, and assemblage variations in relation to lithology. These variations may have been influenced by a number of factors, including global climate, local paleoceanography, changes in sea level, and tectonic uplift along the Alpine Fault. In general, Expedition 317 age assignments relied on nannofossils from the Holocene to middle Pliocene and planktonic foraminifers from the early Pliocene to late Miocene. Both fossil groups were integral for biostratigraphic control from the middle Miocene to Eocene. The following biostratigraphy and resulting interpretations are described in the order that sequences were drilled, from youngest to oldest.

Holocene

The Holocene was not identified biostratigraphically, but based on lithologic evidence, the thickness of Holocene sediments decreased oceanward, from a maximum of 9.9 m at the innermost shelf Site U1353 to 1.2 m at slope Site U1352. Mudline samples from Sites U1352, U1353, and U1354 contained temperate planktonic foraminiferal assemblages consistent with the Holocene highstand and the present-day landward position of these sites relative to the Southland Front.

Pleistocene

Calcareous nannofossil dating provided robust age control through the Pleistocene and contributed to the identification of several intra-Pleistocene hiatuses at all three shelf sites. The hiatuses potentially correspond to seismically mapped sequence boundaries (Fig. F20). Calcareous nannofossil abundance appears to be cyclic. Maximum abundances are thought to be correlated with highstands and lowest abundances with lowstands. This is consistent with the observed lithologic variations. In general, greenish gray sandy marls are associated with high numbers of calcareous nannofossils and warm-water populations, whereas gray sandy muds are associated with low numbers of nannofossils and cool-water populations. The Pleistocene section at slope Site U1352 is expanded relative to the shelf sites, and the hiatuses that were seen at the shelf sites were either absent or less pronounced at Site U1352.

Planktonic foraminiferal assemblages through the Pleistocene sections of shelf sites U1351, U1353, and U1354 are generally characterized by low abundances and small, thin-walled neritic forms typical of deposition under inner neritic conditions (Fig. F24). The low abundances are consistent with an offshore westerly wind pattern, similar to that of the present day. However, higher abundances indicative of deposition under outer neritic or extra-neritic conditions were noted in some samples, especially greenish gray sandy marls. These assemblages included larger temperate forms, whereas interbedded gray calcareous muds were dominated by smaller cold-water forms, including the subantarctic species Neogloboquadrina pachyderma. Similar alternations between temperate and cold-water assemblages were noted throughout the Pleistocene succession. Age-diagnostic planktonic foraminifers that need deep water to complete their reproductive cycle were generally rare, which contributed to the poor planktonic foraminiferal dating through the Pleistocene. Planktonic foraminifers were better represented at slope Site U1352 (Fig. F24), but the temperate species that facilitate a robust correlation with the New Zealand timescale were relatively rare and were generally only found in greenish gray sandy marls. There were few age-diagnostic species in intervals of gray calcareous mud.

Pliocene

The Pliocene/Pleistocene boundary falls within an unconformity at all four sites and is a major correlation surface that falls close to the predicted depth of sequence boundary U13 (Table T2). Biostratigraphic evidence indicates that most, if not all, of the upper Pliocene is missing. The thickness of the Pliocene succession is poorly constrained at the innermost shelf Site U1353, but at middle shelf Site U1354 the middle Pliocene is 50 m thick and the lower Pliocene is at least 185 m thick. At the outermost shelf Site U1351, where the Pliocene is represented by a progradational succession, the middle and lower Pliocene sections are 100 and 580 m thick, respectively. The same progradational succession at slope Site U1352 is 390 and 380 m thick, respectively, for the middle and lower Pliocene sections.

The standard calcareous nannofossil zonation schemes (typically derived from mid to low latitudes) could not be applied to the Pliocene section at any site in spite of the fact that other scientific drilling expeditions at even higher latitudes found the standard zonal markers and were able to apply the schemes. This is attributed to the relatively cold (not warmer than temperate) oceanographic setting of the Canterbury Basin. Age-diagnostic planktonic foraminifers associated with the Globoconella sphericomiozea-puncticulata-inflata group are common throughout the Pliocene section of slope Site U1352 and the lowermost Pliocene section of outer shelf Site U1351. This, in combination with calcareous nannofossils and seismic stratigraphy, provided age control through the Pliocene sections of the shallower shelf Sites U1353 and U1354.

At Site U1351, planktonic abundances increase downhole throughout the lower Pliocene section below 590 m, marking a downhole change from shelfal to slope sedimentation and reflecting oceanward migration of the shelf break over the site. Significantly, a major influx of reworked late–middle Miocene calcareous nannofossils occurs in this same interval. The absence of standard zonal markers in the reworked calcareous nannofossil assemblage indicates that it must have come from a cold, southerly source. This has implications concerning the role of along-strike sediment transport, the focus of which appears to be at the shelf break.

Similar increases in planktonic foraminiferal abundance related to slope deposition and reworking of Miocene calcareous nannofossils were not seen in the Pliocene sections of the shallower shelf sites. In fact, the Pliocene–?upper Miocene section of the most landward Site U1353 is largely devoid of microfossils, except for occasional pulses of calcareous nannofossils and planktonic foraminifers.

Miocene

Upper Miocene sediments were recovered at all shelf Sites U1351, U1352, and U1353, although the dating and subdivision of the Miocene was very poorly constrained at the latter site. The Miocene/Pliocene boundary occurs between sequence boundaries U7 and U6 at all three sites and appears not to fall within an unconformity.

The reworking of calcareous nannofossils noted in the lower Pliocene section of Site U1351 continues through the 204 m thick progradational upper Miocene slope section at the same site, except the age of the reworked material ranges from middle Miocene to Oligocene. The continuity of the upper Miocene section at this site is interrupted by a major hiatus between sequence boundaries U6 and U5, where a gap of ~5 m.y. exists between 5.6 and 10.6 Ma (Table T2).

At slope Site U1352, the progradational upper Miocene section is 220 m thick and overlies a 190 m thick middle Miocene section and a 170 m thick lower Miocene section. Deposition at this site occurred in a lower to basal slope environment under oceanic conditions (Fig. F24). The high abundance of planktonic foraminifers and the presence of bolboformids contributed to reliable dating at this site, although preservation was generally poor, especially in intervals of cemented sandy marl. Calcareous nannofossil assemblages in the Miocene section of Site U1352 include pulses of reworking, but the total amount of reworked material is less than that at Site U1351. This means that approximate calcareous nannofossil ages can be assigned, even without zonal markers of the Reticulofenestra lineage. The major hiatus recognized between sequence boundaries U6 and U5 in the lower upper Miocene section at Site U1351 was also recognized at Site U1352, although it occurs just above rather than below sequence boundary U6. This suggests that the seismic mapping of boundary U6 needs to be reviewed. A significant hiatus was also noted between the middle and upper Miocene at Site U1352, where at least 1.3 m.y. was missing. The same hiatus may also occur in the lowermost part of Site U1353, although the poorly constrained dating at this site reduces the reliability of the correlation. A marked change to warmer planktonic foraminiferal and calcareous nannofossil assemblages occurs in the lower Miocene, below the level of the hiatus. This coincides with the return of standard nannofossil zonal markers in the lower Miocene. Another major hiatus associated with the Marshall Paraconformity was recognized at the base of the Miocene succession, where ~12 m.y. was missing. At ODP Site 1123, on the deep northeast slope of Chatham Rise, ~1000 km north-northeast of Site U1352, a similar amount of time is missing at the Marshall Paraconformity (Shipboard Scientific Party, 1999).

Oligocene to Eocene

Slope Site U1352 was the only site at which Oligocene and Eocene sediments were recovered. Microfossil preservation is generally poor in this interval, but good age control was still possible using calcareous nannofossils and planktonic foraminifers. The very high abundances of planktonic foraminifers throughout this succession are consistent with basin floor deposition under an open oceanic water mass (Fig. F23). The Oligocene/Miocene boundary falls within the Marshall Paraconformity, with a hiatus extending from lower Miocene (18–19 Ma) to lower Oligocene (30.1–32.0 Ma), a gap of ~12 m.y. The Eocene/Oligocene boundary falls within a hiatus of ~2.3 m.y. from the lower Oligocene (32.5–32.9 Ma) to the upper Eocene (35.2–36.0 Ma), based on calcareous nannofossil evidence (Fig. F20). The age at the bottom of the hole is 35.2–36.0 Ma.

Oceanicity

Planktonic foraminiferal abundance provides a measure of the overlying oceanicity and essentially serves as a measure of the distance from land in paleogeographic reconstructions. Test size, diversity, and the composition of the planktonic foraminiferal component of foraminiferal assemblages also serve as proxies for oceanicity.

Oceanicity records from the Expedition 317 sites are markedly different, except in the Pleistocene sections of the three shelf sites, where significant high-amplitude fluctuations in planktonic abundance were noted (Fig. F24). Peaks in abundance in this interval may correspond to highstands and could potentially be correlated across the shelf.

The Pliocene to Miocene succession of landward shelf Site U1353 is characterized by an almost total absence of planktonic foraminifers (Fig. F24), which is unusual even in such a shallow marine environment and is in marked contrast with the continental shelf in the Taranaki Basin, where planktonic foraminifers are relatively common. This difference is attributed to the prevailing westerly wind pattern, which in Taranaki drives planktonics inshore and in Canterbury drives them offshore. Turbid water over the Canterbury shelf, related to high-volume river discharge, may also have contributed to the paucity of planktonics.

The Pliocene–Pleistocene record of oceanicity at shelf Site U1354 is similar to the age equivalent succession at Site U1353, except planktonic abundances are slightly higher (Fig. F24). This is consistent with the site being farther from land and closer to the shelf break.

Planktonic abundances throughout the lower Pliocene to Miocene succession of outermost shelf Site U1351 increase from inner neritic to suboceanic values (Fig. F24). The onset of the downhole increase in the lower Pliocene occurred when the shelf break was located near the site. The subsequent increase in oceanicity below this level suggests the site was located more oceanward of the shelf break and more distal from land. This trend is also reflected in the sizes of tests and the diversity of planktonic foraminiferal assemblages. Second-order high-amplitude perturbations in the oceanicity record of the slope succession may be related to changes in sea level, although more detailed sampling is needed.

The abundance of planktonic foraminifers at slope Site U1352 generally increase downhole from suboceanic values in the Pleistocene to fully oceanic values in the Oligocene (Fig. F24). This is consistent with deposition on the mid- to lower slope and possibly the basin floor in the lowermost part of the succession. It also indicates that the site was always located oceanward of the shelf break.

Benthic foraminifers and paleowater depths

Although the abundances of benthic foraminifers vary, they constitute the major microfossil component within the 150–1000 µm size fraction at all sites. The preservation of benthic foraminifers is generally good in the Pleistocene at all sites, becomes poor to moderate in the Pliocene and Miocene at shelf Sites U1353, U1354, and U1351, and is poor in the Oligocene and Eocene at slope Site U1352.

The Pleistocene assemblage is composed of mainly inner to outer shelfal species at shelf Sites U1351, U1353, and U1354 and outer shelf to upper bathyal species at slope Site U1352. Subtidal to inner shelfal assemblages alternate with mid–outer shelfal assemblages at shelf sites, implying cyclic changes in water depth. General trends of deepening or shallowing can be provisionally correlated with some seismic sequence boundaries (Fig. F25). Pliocene assemblages at the shelf sites are similar to those of the Pleistocene, with marker species suggesting slightly deeper shelfal environments oceanward across the transect. Several benthic foraminifers are also useful for dating, especially in the Pleistocene section at the shelf sites.

In spite of the poor preservation and rarity of earliest Pliocene to latest Miocene benthic foraminifers at slope Site U1352, assemblages suggest a downhole decrease in shelfal species and an increase in upper bathyal marker species. Deep mid- to deep lower bathyal species are the main components in the middle Miocene. Water depths at slope Site U1352 have therefore shallowed from lower bathyal to uppermost bathyal since the middle Miocene as the shelf-slope sediment prism prograded out into the basin (Fig. F25). Early Miocene and Oligocene benthic foraminifers are too rare for reliable estimation of paleowater depths.

Age-depth plots

Age-depth plots (Figs. F26, F27, F28, F29) were created for each site using biostratigraphic datums, New Zealand stage determinations based on foraminiferal assemblages, and paleomagnetic reversal data at Site U1354. At all sites, age control data points were spaced too far apart to determine sedimentation rates with accuracy; however, average sedimentation rates were calculated over the gross intervals between major hiatuses, where the rate appeared to remain constant (dashed lines on the graphs). These rates may be averaged over unresolved hiatuses, so the relatively low numbers may actually represent high rates of sedimentation separated by periods of non-deposition and/or erosion.

The pattern of sedimentation rates across the sites indicates relatively slow deposition during the early to middle Miocene (data mainly from slope Site U1352; Fig. F29), with one or more hiatuses in the late Miocene, dividing this succession from a latest Miocene to Pliocene period of rapid deposition across both shelf sites (data from Sites U1354, U1351, and U1352; Figs. F27, F28, F29).

Note that the benthic foraminiferal datums included in the graphs are in some cases likely to be strongly affected by environmental changes. An example of this effect is the highest occurrence of Hopkinsina mioindex at 1220 m in Hole U1352C (Fig. F29). This datum (3.62 Ma) appears displaced from the rest of the data points in this zone, perhaps because the species occurs only in water depths greater than ~1000 m. This highest occurrence is therefore actually an indication that water depths in the depositional environment reached 1000 m at this horizon.

Hiatuses were identified by the biostratigraphy group based on the occurrence of multiple datums at a single stratigraphic level, as well as on "softer" data such as sudden changes in abundance, preservation, and faunal assemblages. Sometimes these hiatuses corresponded with lithologic changes or surfaces identified in the cores, but more often low recovery meant that unconformity surfaces were not recovered. The exception is Site U1354, where several sharp lithologic changes were recovered in intervals where biostratigraphy identified hiatuses; the good recovery allowed the identification of two unconformity surfaces between 60 and 80 m (Fig. F27), whereas at other sites only one unconformity was definitely identified. Otherwise, the unconformities first identified at Site U1351 were also identified at other sites, particularly a mid-Pleistocene unconformity (two surfaces at Site U1354), an unconformity including the Pliocene/Pleistocene boundary (identified at all sites), and a late Miocene unconformity (or possibly two, identified at Site U1352).

Porosity and backstripping

One of the goals of Expedition 317 was to provide the data necessary to allow quantitative analysis of subsidence and, ultimately, sea level change in the Canterbury Basin using two-dimensional backstripping. In addition to paleoenvironment and age data, backstripping requires an estimate of the history of sediment compaction. Porosity measurements reveal variations in compaction of mixed, largely terrigenous sediments as a function of both burial depth and location across the shelf to the upper slope (Fig. F30).

All sites experience rapid compaction in the upper several tens of meters, suggesting a rapid change from loosely to closely packed grains. Beneath this is a zone in which little porosity loss occurs with depth. The thickness of this zone increases from the middle shelf to the upper slope (Fig. F30). This zone is all but absent at landward shelf Site U1353, where compaction is most rapid as a function of depth. At upper slope Site U1352, two overlapping compaction trends were observed (Fig. F30). The coarser grained facies display rapid cementation and porosity loss, whereas interbedded finer grained sediments remain unlithified to greater depths and lose their porosity more slowly. The improved understanding of sediment compaction and diagenesis that arises from these data will be incorporated into backstripping models.

Geochemistry

The transect of the four coring sites drilled during Expedition 317 provided the opportunity to study pore fluid and sediment geochemistry in contrasting settings, from mid-shelf to upper slope sediments spanning water depths of 85–344 m. This contrasts with the majority of scientific ocean drilling sites, most of which involve coring in water depths from 1500 to 5000 m. Shelf sediments display variability imposed by glacial–interglacial changes in sea level and changes in conditions associated with discontinuities in sedimentation, including subaerial emergence, uplift, and erosion. These conditions introduce complications such as missing section and major changes in variables such as temperature and pressure that affect the ephemeral properties of pore fluids.

Many of the geochemical processes studied in the shipboard geochemistry program are driven by the microbially mediated remineralization of organic matter buried with the sediments. In addition, other processes are seen to advance with increasing burial depth as unstable mineral assemblages are reorganized by dissolution and precipitation. By sampling and analysis at successive core depths, we attempted to piece together the dominant geochemical processes that have taken place in the sediments. The analyses are grouped conveniently by gas, liquid, and solid.

Gas and shallow biogeochemical zones

During Expedition 317, cores at all four sites were routinely sampled for traces of residual dissolved gas, both as a hydrocarbon safety procedure and as a sensitive technique to determine the status of the microbial and thermal degradation of organic matter. Combined with high-resolution water samples (taken in every section of recovered cores), this approach led to profiles covering the biogeochemical zones of sulfate reduction, anaerobic oxidation of methane (AOM), and methanogenesis at all sites in great detail (Fig. F31).

Sites U1351 and U1352 are located farther offshore and show similar geochemical profiles that are distinct from Sites U1353 and U1354. At both sites a classic sulfate–methane transition (SMT) zone is located at ~15–16 m. In sediments below the SMT, sulfate is virtually absent and methane concentrations increase dramatically from background levels in the low ppmv range to >10,000 ppmv. The alkalinity at the SMT maximizes at 10 and 24 mM for Site U1351 and U1352, respectively. The apparent levels of carbon oxidized and the low levels of ammonium and phosphate generated at Site U1351 suggest that sulfate reduction is primarily fueled by AOM, whereas at Site U1352 both AOM and organic matter oxidation are coupled with the removal of sulfate. Residual methane concentrations below the SMT remain constant in the range of 10,000–20,000 ppmv for both sites, approximately equivalent to 4–8 mM of methane per liter of pore volume, which probably represents only 5%–10% of the actual subsurface dissolved gas contents. However, ethane levels (shown by the methane/ethane or C₁/C₂ ratio) vary significantly. C₁/C₂ at Site U1351 shows the expected gradual increase with depth and temperature, but C₁/C₂ ratios at shallow depths are unusually low (400–600) for sediments at such shallow burial depth. This suggests some combination of higher temperatures associated with warmer bottom waters, some removal of sediment at unconformities, preferential loss of methane by anaerobic oxidation, or gas loss from sands during core recovery and sampling. At Site U1352, which is located farther offshore on the slope, gas just below the SMT has a C₁/C₂ ratio of 16,000, which is lower than most deepwater coring sites. The average C₁/C₂ ratio decreased by three orders of magnitude to 10–60 at the bottom of Hole U1352C at 1927 m. At an apparent discontinuity near 1395 m, only very low gas content was found (40 ppmv methane) and C₁/C₂ dropped as low as 7, mainly from the near absence of methane. Below the discontinuity the gas resumed the normal trend of residual methane contents (>10,000 ppmv). At both Sites U1351 and U1352 the higher hydrocarbons (C₃–C₅) were detected in increasing amounts with depth, apparent products of the low-temperature alteration of organic matter in the deeper cores.

Sites U1353 and U1354 are located on the shelf in water depths of 85 and 120 m, respectively. Both sites are characterized by the absence, or low levels, of methanogenic activity in the uppermost 150 m, leading to low methane concentrations. At Site U1353 no hydrocarbons above background levels were detected, and sulfate remains close to seawater concentration with the exception of slightly decreased sulfate in a lower salinity interval at 20–70 m. No SMT is apparent, suggesting that either (1) methanogenesis did not occur in the sediments, (2) previously generated methane was lost when the shelf was emergent, or (3) methane was oxidized when sulfate was replenished by diffusion after a subsequent sea level rise. Alkalinity never rises above 6.5 mM and declines gradually with depth.

Hydrocarbon monitoring at Site U1354 showed two peaks in methane, a small peak (23 ppmv) at 33–75 m, and a large peak below ~200 m, where methane increased to >20,000 ppmv. In both cases methane begins to increase when sulfate drops to zero, and the shallow methane peak disappears when sulfate reappears in the cores. These alternations of methanogenesis appear to be related to changes in sedimentation rate. The upper and lower methane zones correspond to periods of rapid sedimentation (200–400 m/m.y.) in which sulfate was depleted and methane was generated. The intermediate interval (60–200 m) corresponds to periods of slower sedimentation (<50 m/m.y.) when pore waters could be replenished with sulfate by diffusion from overlying seawater.

Interstitial water geochemistry

Interstitial water (IW) was squeezed from sediments through most of the sedimentary sequence for the three shelf sites, but only in the uppermost 1400 m of slope Site U1352. At this site, pore water yield decreased from ~3–5 mL/cm of whole round squeezed in the shallower sediments to <1 mL/cm below 1200 m, reflecting progressive compaction, porosity and water loss, and the general sediment to rock transformation. Pore waters in the Canterbury Basin have quite variable properties depending on site location on the transect.

Below the sulfate reduction zone calcium always increases with depth, but at different rates and to different values (Fig. F32). At Sites U1353 and U1354, the increase occurs mainly in the uppermost 200 m and reaches only moderate values (<20 mM), whereas at Sites U1351 and U1352 the calcium increase is noticeably sharp (at ~250 and 600 m, respectively) and reaches values of 30 mM and higher at the base of Site U1351. These variations are likely related to carbonate diagenesis, and possibly contributions from deeper basinal brines, and are broadly consistent with dissolution of carbonates and the poor preservation of microfossils. More particularly, there is evidently some exchange of magnesium for calcium in the dissolution and reprecipitation of carbonates during diagenesis at Sites U1351 and U1352, as indicated by the greater declines in the Mg/Ca ratio and the higher carbonate content at these sites.

Alkalinity, mainly as bicarbonate ion produced by oxidation of organic matter, always decreases with depth below the sulfate reduction zone and generally reaches values <3 mM below 200 m, although this transition, together with the increase in calcium, occurs deeper at ~500 m at slope Site U1352.

Chloride and salinity generally co-vary, but there are other controls on sodium content. The salinity and chloride content of the pore waters remains constant and near seawater composition throughout slope Site U1352, except for the deepest sample, which has a significantly lower chloride content. At Site U1351 the chloride becomes more concentrated than seawater between 50 and 200 m and again below 900 m. The highest salinity and chloride values are found at Site U1354, where they increase throughout the sequence and reach values of 3.8 and 695 mM, respectively. The deepest samples from Site U1353 are also the most saline, but this site is also characterized by a salinity minimum in the uppermost 150 m that reaches 2.4 mM or 70% of seawater at ~50 m. This less saline lens affects the pore water composition of several other ions, including sodium, potassium, and magnesium (Fig. F32). The presence of this less saline lens could be explained by either modern intrusion of meteoric water from land or by the historic remains of freshwater, emplaced when the shelf was emergent and now being slowly replaced by the downward diffusion of seawater. This less saline water lens is not present at the more seaward Sites U1354 and U1351. Water depths are 85 m at Site U1353, compared with 110 m at Site U1354 and 122 m at Site U1351. Global sea level was about 125 m below today's sea level at the last glacial maximum about 20,000 y ago, so both Sites U1353 and U1354 should have experienced periods of complete emergence. Therefore, the lack of less saline water at Site U1354 and its presence at Site U1353 are more likely explained by the modern intrusion of meteoric water from land, rather than by the historic remains of freshwater emplaced when the shelf was emergent.

Potassium concentrations decrease with depth at all four sites, starting from slightly higher than seawater values of ~11 mM in the shallowest samples (Fig. F32). The decrease is rapid to <6 mM within the uppermost 150 m burial at the shelf sites but is much more complex at the slope site, with an initial decrease in the sulfate reduction zone, an increase back to seawater values, and finally a steady decrease to <4 mM below 800 m (Fig. F32). The decrease in potassium, and to some extent sodium, is likely related to diagenetic glauconite precipitation, which is a common mineral at all four sites and usually appears around the depth of the potassium drawdown.

Silica and silicon concentrations are always <1 µM and generally show rapid fluctuations in abundance in the uppermost 100 m. At the slope site this variability persists throughout the section with IW samples, but silica and silicon are more consistent and less abundant below 150 m. At Site U1351 silicon concentration levels are ~400–500 µM, whereas they are ~200 µM at nearer shore sites. This suggests that silica mobilization and precipitation is more active nearer to shore, consistent with the lesser carbonate and greater clastic component at these sites.

Lithium concentrations generally increase with depth at all four sites, with very smooth profiles to values of <100 µM at Sites U1353 and U1354 and more complex patterns and higher overall concentrations at the two more distal sites. The lithium increases may be caused by dehydration reactions removing lithium from clay interlayer exchange sites and in some cases are related to lithologic changes.

Barium concentrations increase rapidly in the shallowest samples (<40 m), and then generally remain steady until >250 m, below which they increase. At Sites U1353 and U1354 this increase never exceeds 10 µM, but at Site U1352 barium concentration is much higher (>20 µM) in the samples below 500 m. Site U1351 has intermediate barium concentrations in the deeper sediments (5–10 µM) but higher abundance (as high as 23 µM) in the four shallowest samples, which so far remains unexplained.

Boron concentrations also increase with depth at all four sites, but to different values. Boron reaches ~0.6 µM in the deepest sample (586 m) at Site U1353, ~1.25 µM at 320 m at Site U1354, and maximum values of 4–5 µM at Sites U1351 and U1352. Thus there is a trend of higher boron farther offshore and deeper in the cores, which may be consistent with a diagenetic opal-A/opal-CT transition and microbial degradation of organic matter.

Sediment geochemistry

The sediments cored at Site U1351 show marked differences in geochemistry with depth, with higher carbonate, higher nitrogen, and lower sulfur above 200 m (Fig. F33). Organic carbon contents range from 0.3 to 1.5 wt%, with more frequent higher values in the uppermost 200 m. Pyrolysis characterization suggests the organic matter is dominated by degraded higher plant debris.

Analyses of sediment samples at Site U1352 also distinguished the clay-rich lithologic Unit I from the carbonate-dominated Unit II. Organic carbon content was generally low (<0.6 wt%), with only a few samples having >1% total organic carbon (TOC) (Fig. F33). The character of the organic matter changes from relatively labile volatile material in the shallower sediments to more stable proto-kerogen downhole, with evidence for increasing thermal maturity at total depth. The organic matter appears to be mainly terrestrial plant in origin.

At Site U1353 the average carbonate content is low (0.5–2 wt%) and decreases with depth (Fig. F33). The decrease of TOC over the uppermost 100 m can be correlated with the intervals of increased alkalinity and decreased sulfate and might represent active biological oxidation. Pyrolysis characterization of organic matter suggests a major contribution of terrestrial plants, whereas C/N ratios from elemental analysis are consistent with a significant marine influence.

Only 18 sediment samples were analyzed at Site U1354 for carbonate content and by the elemental analyzer due to time constraints at the end of the expedition. Calcium carbonate contents range from 1.3 to 52 wt% in the sediments analyzed down to burial depths of 81 m. Organic carbon ranges from 0.02 to 1.1 wt%, with the highest value at 50 m (Fig. F33). The ratio of TOC to total nitrogen generally decreases with depth, with the exception of some of the high carbonate samples in the 73–76 m depth interval.

Microbiology

Microbiological studies on sediment cores collected during DSDP, ODP, and IODP have demonstrated the presence of microbial communities in deeply buried marine sediments down to >1600 mbsf. The Canterbury Basin is a promising place to expand our knowledge of the deep biosphere in a complex setting, representing the history of life under a variety of environmental constraints. The basin is heavily influenced by the input of terrestrial organic matter and is therefore an excellent end member environment to complement the marine settings that have been studied during previous drilling expeditions.

The Expedition 317 shipboard microbiology program included collection of samples for onshore microbiological investigations, assessment of the degree of seawater contamination of the microbiological sediment samples, and testing of a new cell counting technique. Three sites were sampled for microbiology investigations (Sites U1351, U1352, and U1353). Eleven whole-round samples were taken between 1630 m and the bottom of Hole U1352C at 1928 m for microbiological and organic geochemical characterization of the in situ microbial communities. These samples will be studied in more detail onshore and could potentially extend the maximum known depth of habitable sediments.

A new cell counting technique, recently developed by Morono et al. (2009), was also tested. The procedure proved to be highly efficient, and discriminative detection and enumeration of microbial cells in sediments using diluted (1%) hydrofluoric acid was achieved at Site U1351. The greatest abundance of cells was found in the near-surface sample, which contained 1.08 × 10⁸ cells/cm³, and the smallest number of cells was found at 930.9 m, containing 1.34 × 10⁵ cells/cm³. Generally, the total number of cells decreased rapidly with depth in the uppermost 4 m of sediment. This depth profile follows a trend observed at other ODP sites, but the absolute numbers of prokaryotes are lower than the average numbers for all previously examined sites, particularly below 4 m.

The contamination tests on board were performed using particulate tracers and provided confirmation that uncontaminated samples can be recovered and that there is no difference in contamination produced by the three types of coring (advanced piston corer [APC], XCB, and RCB). Nevertheless, the dispersion of these tracers was found not to be homogeneous along the core liner, and the microsphere concentrations were found to be diluted during RCB coring.

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