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doi:10.2204/iodp.pr.302.2005
SYNTHESIS
Site Overview
Cores were recovered in five holes across three sites (Holes M0002A, M0003A, M0004A, M0004B, and M0004C) (Tables T1, T2, T3), with a total recovery of 68.4%, which precluded the complete recovery of the entire sediment drape, a key objective (Fig. F4). The first hole at Site M0001 was abandoned because a bottom-hole assembly (BHA) was lost (see "Site Operations" in "Operations"). Logging was attempted in two holes and data were collected over a 160 m interval in Hole M0004B.
The ACEX sites are situated within 15.7 km of each other on seismic Line AWI-91090 and have been treated as a single site because of the internally consistent seismic stratigraphy across that distance (Fig. F2). A limited amount of site-to-site correlation was conducted, based primarily on physical property data (gamma ray attenuation bulk density, magnetic susceptibility, P-wave velocity, and electrical resistivity) generated using a multisensor core logging system. Site correlation was also aided by lithologic descriptions and high-resolution geochemical pore water measurements of chiefly ammonia concentrations and alkalinity. In terms of recovered stratigraphy, the bulk of material was provided by Hole M0002A for the upper half of the 428 m long stratigraphic record and Hole M0004A for the lower half, with correlation made possible by a short overlap between the two holes. The other holes recovered multiple portions of the upper ~30 m and allowed construction of a composite depth scale and spliced record for this short interval.
Preliminary Scientific Assessment
The overall goal of ACEX was to study Arctic paleoceanography in order to understand this region's past climate and its impact on Earth's climate over the Cenozoic, with particular emphasis on the change from the "greenhouse" world of the Eocene to the "icehouse" world of today.
The prospectus outlined the methods to achieve this goal. The primary method was to apply a well-known and effective technique for complete core recovery: continuous piston core and extended core barrel sampling in multiple holes at one site. This technique results in a continuous stratigraphic record. ACEX was not able to apply this method during Expedition 302, as no sites in ACEX were multiply cored (Table T1; Fig. F4). In addition, single hole penetrations suffered from relatively low recovery. The average core recovery for all holes was 68.4%; below 270 m (~47 Ma) to total depth at 428 mbsf, the recovery dropped to 43.1%. This lower recovery interval spanned two of the most important ACEX events: the Azolla and the early Eocene Thermal Maximum (EETM), resulting in incomplete records for both.
The early results of ACEX show that further analyses of the sediment and basement cores will contribute to five of the seven major paleoceanographic objectives and both of the tectonic objectives. The degree to which advances are made depends on the level of detail that can be extracted from the sediment record. Because of the low core recovery, an average of one-third of this detailed record is missing, thus reducing the chances for fully meeting each objective.
The predicted lithologies were encountered. Stratigraphically, the ACEX sites show a major hiatus spanning the transition from the Neogene to the Paleogene. The overall age span of the sediment section was greater than predicted by a few million years. The overall impact of these two factors is limited. The hiatus means that paleoceanographic analyses over this missing interval cannot occur, but interpretation of the overall time and causal mechanisms will contribute to furthering our understanding of the tectonic evolution. The longer age interval will allow us to interpret the paleoclimate conditions during the EETM at a position close to the North Pole.
Among the seven specific paleoceanographic objectives, results from ACEX will be used to determine the history of ice rafting and sea ice; study local (e.g., Svalbard) versus regional ice sheet development; reconstruct the density structure of surface waters, the nature of North Atlantic conveyor, and the onset of Northern Hemisphere glaciation; make contributions to the investigation of the development of the Fram Strait and deepwater exchange between the Arctic and the world ocean; and determine the history of biogenic sedimentation. Because one of the hiatuses in the sediment record may span the Pliocene, it is possible that ACEX results may not be useful in the study the land-sea links and the response of the Arctic to Pliocene warm events. Also, the lack of a carbonate stratigraphic record precludes study of the timing and consequences of the opening of the Bering Strait. Biogenic carbonate occurs only rarely and occasionally in the upper 18 m of the sediment column. The disappearance of carbonate occurs together with a decrease in pH and alkalinity, suggesting that the lack of cocolithophorids, calcareous foraminifers, and ostracodes in deeper sediments is caused by dissolution.
ACEX results partially addressed the two tectonic objectives. The regional unconformity was penetrated but not sampled except for a small bag sample. Fossils from this sample constrain timing of the initiation of rifting to between 80 Ma and the oldest age of the sediment overlying the unconformity at 58 Ma.
Early results reveal that the upper sediments hold a record of sea-ice distribution in the Arctic Ocean well into the middle Miocene. The situation is different in older, underlying cores where dark, organic-rich sediments contain abundant diatoms, ebridians, silicoflagellates, and dinoflagellate cysts, indicating a middle Eocene age and an environment partly characterized by ice-free, warmer surface ocean waters. Isolated pebbles, interpreted as dropstones, were observed downhole to ~239 mbsf, suggesting the presence of at least seasonal ice throughout most of the middle Eocene.
Abundant megaspores of the hydropterid fern Azolla occur at the early/middle Eocene boundary, suggesting strongly reduced surface water salinity or perhaps even a brief episode of freshwater conditions at the surface. It is yet not known if the Azolla spores represent an indigenous signal, indicating fresh to nearly fresh surface waters, or if they have been transported into a marine Arctic basin from neighboring freshwater systems. However, the sporadic and rare presence of radiolarians suggests that the Arctic's surface water salinities indeed were reduced throughout the Eocene interval containing biosilica. Biosilica is not preserved before the late early Eocene. The dinoflagellate species Apectodinium augustum is abundantly present at ~380 m in pyrite-rich mudstones, indicating that the EETM interval was partly recovered. During this thermal maximum, the Arctic Ocean experienced surface temperatures on the order of 20°C.
Lithostratigraphy
The lithostratigraphy of the Lomonosov Ridge sites is described in terms of four units (Fig. F5). Recovered sediments, ranging in age from Holocene to Late Cretaceous (0–428 mbsf), are dominated by lithogenic material. With the exception of sandy lenses, the dominant terrigenous component of all lithologic units is fine grained, ranging from clays to silty muds indicative of predominantly low energy marine environments. The upper ~220 mbsf comprises soft to hard silty clay with colors varying from light brown to olive green to gray (Unit 1). Isolated pebbles are present throughout Unit 1, with the deepest pebble observed in Unit 2 (239.34 mbsf). This may indicate the presence of at least seasonal sea ice as early as the middle Eocene. Lithologic unit changes do not coincide with hiatuses interpreted on the basis of biostratigraphy. However, a major hiatus is likely within Core 302-M0002A-46X near the boundary of Subunits 1/5 and 1/6.
Below ~220 mbsf, the sediments change from biosiliceous silty clay to biosiliceous ooze encompassing an interval of ~93 m (Unit 2). The biosiliceous sediments overlie an interval of hard silty clay to mudstone (Unit 3), which, at ~410 mbsf, rests unconformably on Campanian marine sands, sandstone, and mudstone (Unit 4).
Micropaleontology
Prior to ACEX, information about microfossil contents in central Arctic Ocean cores was limited to observations made in short piston and gravity cores. These cores held records of variable and discontinuous abundances of calcareous nannofossils, planktonic and benthic foraminifers, ostracodes, and dinoflagellate cysts (e.g., Aksu et al., 1988; Scott et al., 1989; Gard, 1993; Cronin et al., 1994; Ishman et al., 1996; Matthiessen et al., 2001). A single core from the Alpha Ridge contained middle Eocene diatoms and silicoflagellates (Bukry, 1984; Ling, 1985). Before ACEX, no accurate knowledge existed about which biostratigraphically useful microfossil groups may be encountered at depth. Therefore, expertise representing all above microfossil groups participated during the offshore phase. ACEX core catcher samples were also systematically searched for radiolarians and fish debris.
Biogenic carbonate is missing with the exception of the upper 18 m. Dinoflagellate cysts provide the bulk of available biostratigraphic information in the upper ~170 m.
A 23 m thick interval below ~170 mbsf appears to be completely devoid of microfossils, referred to as the "spooky" interval by the offshore micropaleontologists. The spooky interval separates the overlying middle Miocene from the underlying middle Eocene and presumably preserves some of the lower Neogene and upper Paleogene sections. Dinoflagellate cysts, diatoms, ebridians, and silicoflagellates are common to abundant in the middle Eocene section, which ends in an interval with megaspores of the freshwater hydropterid fern Azolla at the lower/middle Eocene boundary (~306 m). Biosilica is not preserved prior to the late early Eocene (~320 m).
The (sub)tropical dinoflagellate species A. augustum occurs abundantly at ~380 m, indicating that the Paleocene/Eocene boundary and the associated carbon isotope excursion interval was at least partly recovered.
Benthic foraminifers indicate that the lower Eocene through upper Paleocene sediments were deposited in shallow-marine, neritic to inner-neritic environments.
Sedimentation Rates
Biostratigraphy provided age/depth information that will be useful when developing the age model for the ACEX sites. Paleomagnetic data are being acquired postcruise, which will be amalgamated with the biostratigraphic data to provide an age model. These data will be presented in the Expedition 302 Results volume. Among the biostratigraphy, dinocysts provide the bulk of the Neogene biostratigraphic data. In the Eocene, diatoms and silicoflagellates were added to the dinocyst data set. The general structure of the biostratigraphic age/depth point distribution shows two distinct intervals, both having rates on the order of 1–3 cm/k.y. (10–30 m/m.y.), namely a Pleistocene to middle Miocene interval and a middle Eocene to uppermost Paleocene interval. Presently we lack age information for a ~23 m thick interval separating the two intervals. However, the distribution of presently available biostratigraphic age/depth points clearly suggests the presence of a major hiatus separating the Neogene and Paleogene intervals. The precise extent of this hiatus and its precise location in the stratigraphic column is presently unknown. Another major hiatus appears to separate upper Paleocene from the underlying Campanian sediments.
Petrophysics
Petrophysical measurements performed during the offshore component of ACEX included downhole wireline logging; nondestructive whole-core measurements of bulk density, compressional P-wave velocity, resistivity, and magnetic susceptibility; and discrete measurements of shear strength and moisture and density.
Downhole Wireline Logging
Downhole logging was attempted in both Holes M0004A and M0004B. A 160 m interval was successfully logged in Hole M0004B with two upward passes, with the second pass crossing the seafloor (see "Site M0004 (Shotpoints 3006 [A, B] and 3004 [C] on Line AWI-91090)" in "Operations"). The tool string comprised the Natural Gamma Ray Spectrometry Tool (NGT), Formation MicroScanner (FMS), Borehole Compensated Sonic (BHC) tool, and Scintillation Gamma Ray Tool (SGT).
The calliper logs from the FMS (two per pass) provided a method for assessing the borehole condition (diameter and rugosity). The bit outer diameter is 9 inches and it was observed that, for much of the formation, the hole diameter was under gauge and narrowed significantly between 75 and 90 mbsf, at 155 mbsf, and again between 180 and 184 mbsf. Given the narrow borehole diameter, the FMS pads should have made contact with the borehole wall for the entire length of the logged section. The caliper logs indicated that the borehole conditions for the complete logged section were good and nowhere did the borehole appear washed out to the degree that it would adversely affect the tool response. This judgement is supported by a favorable comparison of parameter magnitudes between passes. The depth match between the logging passes is good (being for the most part less than much as 2.6 m at ~155 meters below rig floor (mbrf) and improves again in the bottom of the hole. This offset was removed by depth matching the passes.
Multisensor Core Logger and Discrete Physical Property Measurements
Downhole variations in density, P-wave velocity, and magnetic susceptibility highlight a number of prominent stratigraphic changes that exist at all sites and correlate well with observed seismic reflectors. The stratigraphic similarities among the sites allowed a single composite section to be constructed (Fig. F6A, F6B).
Compositionally, the upper 220 m of sediment recovered from Lomonosov Ridge is predominantly silty clay (lithologic Unit 1). In contrast to deep marine sediment sequences, this interval does not exhibit a single, clearly defined consolidation pattern with depth. The upper ~20 mbsf shows first order increases in both density and velocity that appear to arise from normal consolidation processes (Fig. F7A, F7B). Throughout this upper unit, well-defined decimeter-scale variations in density, velocity, and susceptibility occur in phase.
Between 70 and 100 mbsf, there is a shift away from the high-amplitude variation in magnetic susceptibility that is a characteristic feature of the sediments below ~20 mbsf. A subtle first-order reduction in the magnetic susceptibility of the sediments begins at 100 mbsf and continues to ~160 mbsf. This reduction may result from gradual compositional changes occurring through this interval.
A noticeable decrease in all petrophysical properties measured on the multisensor core logger (MSCL) occurs at ~166 mbsf and accompanies the transition from predominantly olive-green sediments into those characterized by a more yellowish to brown hue at the Unit 2/3 boundary. One of the most prominent changes is a large decrease in P-wave velocity at ~198 mbsf, marking the transition into a biosiliceous unit. The upper part of this biosiliceous unit is largely composed of pyritized diatoms and is reflected in the petrophysical data as a low-velocity, high-density interval. At ~220 mbsf, density decreases sharply from 1.7 to 1.3 g/cm3 without a noticeable change in the P-wave velocity and is associated with the transition from a biosiliceous silty clay unit into a biosiliceous ooze (Unit 2).
Large gaps in core recovery occur from ~220 to ~350 mbsf. An increase in density through this interval could be the result of normal consolidation or a reduction in the biosiliceous contribution to the matrix material. Below ~370 mbsf and the end of the petrophysical record at ~410 mbsf, bulk density fluctuates between 1.6 and 2.1 g/cm3. Throughout this interval, large amounts of pyrite are found in the core catcher samples. Peaks in susceptibility (>5 x 10–3 SI) and density (>3 g/cm3) indicate the presence of dense material that is probably of diagenetic origin. The deepest cores recovered from Hole M0004A, documenting the transition through sandstone and mudstones and into basement, were too short and disturbed to be run on the MSCL.
Based on undrained shear strength measurements made on the ends of recovered core sections, the Lomonosov Ridge sediments have a low consolidation index (~0.1) (Fig. F8), suggesting underconsolidation with the exception of specific intervals. At ~55 mbsf, a single measurement indicates a highly overconsolidated zone that overlies seemingly normally consolidated material. Below 155 mbsf, the consolidation ratio becomes slightly elevated and remains high until 193 mbsf where the resolution of the measurement device was suddenly exceeded.
In Situ Temperature Measurements
In situ temperature was measured during coring operations using the British Geological Survey (BGS) and Adara temperature tools (Fig. F9). The mudline temperature was recorded on all runs and varied between tools. An attempt to normalize the in situ measurements was made by using the average Adara determined mudline temperature and adjusting all in situ measurements to this baseline value. The average gradient calculated using all available data points is 43.2°C/km.
Chemistry
Significant features in the shipboard pore water chemistry profiles describe three geochemical environments: shallow carbonate dissolution, deep sulfate reduction, and shallow ammonium oxidation (Fig. F10).
Lithologic and micropaleontogical descriptions of sediment note a general absence of primary carbonate below ~16 mbsf, where pH and alkalinity drop below 7.4 and 2.5, respectively. This means that pore waters near this depth are more corrosive to carbonate tests than the overlying sediment or water column. Carbonate tests may have dissolved when they were buried to the depth of these corrosive pore waters.
High alkalinity below 200 mbsf suggests that chemical reactions are adding substantial amounts of HCO3– at depth without accompanying H+. The likely candidate is sulfate reduction of organic carbon:
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2CH2O(s) + SO42–(aq) => 2HCO3–(aq) + H2S(aq).
Black sediments (Unit 5) were rapidly deposited below 200 mbsf. These sediments host abundant pyrite and lie beneath dark banded intervals that may be composed of other iron sulfide minerals. Organic matter in the black sediment probably reacted with dissolved SO42– since the time of burial, producing abundant H2S and, ultimately, iron sulfide minerals.
A peak in alkalinity (at ~6 meters composite depth [mcd]) coincides with a sharp steady rise in NH4+. The peak in alkalinity supports the interpretation that some chemical reaction is producing HCO3– without accompanying H+. The NH4+ profile further suggests that upward diffusing NH4+ drives this reaction.
Microbiology
Sampling for microbiological analyses was conducted at fairly regular depth intervals from the surface (7 mbsf) to near basement (398 mbsf) with a notable gap between 169 and 241 mbsf. A total of 21 samples were preserved for enumeration of micro-organisms to provide estimates of subsurface biomass. Nineteen samples were stored anaerobically for the purpose of shore-based cultivation studies. A subset of samples (18) was stored at –51°C for DNA extraction and subsequent microbial community characterization. Finally, 10 samples were stored at –51°C for lipid biomarker analysis.