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doi:10.2204/iodp.proc.302.101.2006

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% (Fig. F5). The first hole at Site M0001 was abandoned because a bottom-hole assembly (BHA) was lost (see “Site Operations” in the “Sites M0001–M0004” chapter). Logging was attempted in two holes and data were collected over a 153 m open-hole interval in Hole M0004B.

Expedition 302 sites are situated within 15.5 km of each other on seismic Line AWI-91090 and are interpreted 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 the MSCL. 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 20 mbsf and allowed construction of a composite depth scale and spliced record for this short interval.

Scientific assessment outcomes

The overall goal of Expedition 302 was to study Arctic paleoceanography in order to understand this region's past climate and its impact on Earth's climate during the Cenozoic, with particular emphasis on the change from the “greenhouse” world of the Eocene to the “icehouse” world of today.

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. This method was not applied during Expedition 302, as no sites were multiple cored (Table T1; Fig. F5). 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%. Unfortunately, low core recovery plagued recovery of two important Expedition 302 events: the Azolla and the Paleocene/​Eocene Thermal Maximum (PETM), resulting in incomplete records for both.

The early results of Expedition 302 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.

Generally, the lithologies anticipated in the proposal to IODP were encountered. The overall age span of the sediment section recovered was longer than predicted by a few million years. A major hiatus occurs in the section that spans the transition from the Neogene to the Paleogene. The hiatus means that paleoceanographic analyses over this missing interval cannot occur, but interpretation of the overall time and causal mechanisms will contribute significantly to furthering our understanding of the tectonic evolution and resulting depositional environment. The longer time interval will allow us to interpret the paleoclimate conditions during the PETM at an important geographic position for climate studies close to the North Pole.

Among the seven specific paleoceanographic objectives, scientific results from Expedition 302 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 the 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 Ocean and the World Ocean; and determine the history of biogenic sedimentation. The lack of a carbonate stratigraphic record precludes study of the timing and consequences of the opening of the Bering Strait. Biogenic carbonate is present only rarely and occasionally in the upper 19 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.

Expedition 302 results partially address the two tectonic objectives. The regional unconformity was penetrated but not well 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 also 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.

Abundant megaspores of the hydropterid fern Azolla are present at the lower/​middle Eocene boundary, suggesting strongly reduced surface water salinity or perhaps even an episode of freshwater conditions at the surface. The sporadic and rare presence of radiolarians suggests that the Arctic’s surface water salinities indeed were reduced throughout the Eocene interval that contains biosilica. Biosilica is not preserved before the upper lower Eocene. The dinoflagellate species Apectodinium augustum is abundantly present at ~380 m in pyrite-rich mudstones, indicating that the PETM interval was partly recovered. During this thermal maximum, the Arctic Ocean experienced surface temperatures on the order of 20°C based on early results using TEX86 paleothermometry (Sluijs et al., submitted).

Lithostratigraphy

The lithostratigraphy of the Lomonosov Ridge sites is described in terms of four units (Fig. F6). 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 siliciclastic component of all lithologic units is fine grained, ranging from clays to silty muds. Thin millimeter- to centimeter-thick layers of sand are present down to ~198 mbsf. 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. A major hiatus occurs at the boundary of Subunits 1/5 and 1/6 at 198.13 mbsf.

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). Results from geochemistry demonstrate that the top of Unit 3 (313–350 meters composite depth [mcd]) holds abundant authigenic silica altered from biogenic opal.

Micropaleontology

Prior to Expedition 302, information about microfossil content 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 Expedition 302, no accurate knowledge existed about which biostratigraphically useful microfossil groups would be encountered at depth. Therefore, expertise representing all possible microfossil groups were invited as science party members. Expedition 302 samples were also systematically analyzed for fish debris.

One of the most striking results is that biogenic carbonate is almost completely missing from the sediment with the exception of only the upper few meters. Dinoflagellate cysts provide the bulk of available biostratigraphic information.

A 23 m thick interval below ~170 mbsf appears to be nearly devoid of microfossils. This interval is interpreted to be of middle Miocene age, based on magnetobiostratigraphy. 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 present 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 environments.

Sedimentation rates

Biostratigraphy and magnetostratigraphy were used to construct the age model. 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 1–2 cm/k.y. (10–20 m/m.y.), namely a Pleistocene to middle Miocene interval and a middle Eocene to uppermost Paleocene interval.

A major hiatus separates the upper Paleocene from the underlying Campanian sediments, which presumably marks the boundary between underlying sedimentary bedrock and the overlying sediment drape on the ridge. Another major hiatus occurs at 198 mbsf, where an interval representing a major portion of the lower Miocene, the Oligocene, and the upper Eocene is missing.

Petrophysics

Petrophysical measurements performed during Expedition 302 included downhole wireline logging; nondestructive whole-core measurements of bulk density, compressional P-wave velocity, resistivity, natural gamma radiation, and magnetic susceptibility; and discrete measurements of shear strength, moisture and density, thermal conductivity, and color reflectance.

Downhole wireline logging

Downhole logging was completed in Hole M0004B using the Natural Gamma Ray Spectrometry Tool (NGT), Formation MicroScanner (FMS), Borehole Compensated Sonic (BHC) tool, and Scintillation Gamma Ray Tool (SGT). A 153 m open-hole and 65 m in-pipe interval was successfully logged, providing in situ measurements of P-wave velocity, resistivity, and natural gamma radiation through lithologic Subunits 1/3 to 1/6.

The caliper logs from the FMS (two per pass) provided a method for assessing the borehole condition. 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. The caliper logs indicated that the borehole conditions for the complete logged section were good and free of any washed-out materials.

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. F7).

Compositionally, the upper 220 m of sediment recovered from the Lomonosov Ridge is predominantly silty clay (Unit 1). The upper ~20 mcd shows first-order increases in both density and velocity that appear to arise from normal consolidation processes (Fig. F8). Throughout this interval, well-defined decimeter-scale variations in density, velocity, and susceptibility occur in phase. Below ~20 mcd, a noticeable drop in the shear strength of sediments occurs, remaining low to ~40 mcd, where moisture and density and MSCL measurements indicate an increase in the bulk density of sediments (Fig. F9). Large-amplitude variations in bulk density, P-wave velocity, and magnetic susceptibility characterize the bulk of Unit 1 sediments below ~40 mcd.

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 noticeable decrease in all petrophysical properties measured on the MSCL occurs at ~168 mbsf and accompanies the transition from predominantly olive-green sediments into those characterized by a more yellowish to brown hue at the Subunit 1/3 to 1/4 boundary. One of the most prominent changes is a large decrease in P-wave velocity at ~198 mbsf, marking the transition into Subunit 1/6, which is characterized as a silty clay having relatively high TOC and pyrite concentrations. 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 the pyrite-rich 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 mirrors changes in the biosiliceous contribution to the matrix material. Below ~370 mbsf, spurious peaks in susceptibility (>5 × 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 mudstone and into basement, were too short and disturbed to be run on the MSCL.

In situ temperature measurements

In situ temperature was measured during coring operations using the BGS and Adara temperature tools. Postcruise processing is required to determine equilibrated in situ temperatures. The mudline temperature was recorded on all runs and varied between tools. A preliminary 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, using four of the five measurements, is 30.5°C/km.

Chemistry

Shipboard pore water chemistry profiles suggest three geochemical processes: 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 mM, respectively. Assuming constant dissolved Ca²⁺, this means that pore waters near this depth are more corrosive to carbonate tests than the overlying sediment or water column. Carbonate tests may dissolve when buried in these corrosive pore waters.

The alkalinity inflection at ~200 mbsf suggests chemical reactions are adding substantial amounts of HCO₃^– at this depth without accompanying H⁺. The likely candidate is sulfate reduction of organic carbon:

2CH₂O(s) + SO₄^2–(aq) → 2HCO₃^–(aq) + H₂S(aq).

Black sediments (Unit 2) 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, particularly at ~200 mbsf, dissolved SO₄^2–. This reaction may have produced abundant H₂S in the past and, ultimately, iron sulfide minerals.

A peak in alkalinity (at ~6 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 deoxyribonucleic acid extraction and subsequent microbial community characterization. Finally, 10 samples were stored at –51°C for lipid biomarker analysis.

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