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doi:10.2204/iodp.proc.302.104.2006

Geochemistry

Shipboard results

Chemical profiles constructed from the pore waters of deep marine sediment are typically characterized by smooth changes in concentration with respect to depth because of diffusion. Moreover, identical pore water profiles are usually found in adjacent deep-sea drill holes because of diffusion and similar sediment composition (e.g., D’Hondt, Jørgensen, Miller, et al., 2003). Because sediment properties are discussed in other chapters, all pore water samples from all holes have been placed on a common depth scale (Table T39) and they are discussed as if they came from a single hole. It is noted, though, that the common depth scale was partly revised using the pore water ammonium profile, given the above assumptions.

Across several shallow depth intervals, adjacent whole-round and Rhizone samples were taken to evaluate the merits of the latter technique. The precision for alkalinity and ammonium measurements is fairly high. For these two species, no significant difference was found between samples collected by the two techniques (Table T39; Fig. F38). Thus, samples collected by the two methods are discussed together. In sediment from above 15 mcd, Rhizone samplers almost always collected pore water faster than 0.7 mL/h during a 5 h period. This rate dropped to <0.3 mL/h at 43 mcd, and Rhizone samplers were not used below this depth.

Salinity varies between 35 and 40 ppt (Table T39). In general, salinity decreases with depth, at least to 250 mcd. More subtle changes may occur downhole or in sediment deeper than 250 mcd, but this cannot be assessed with the shipboard data, which suffers from low resolution and low precision. Major changes in the laboratory temperature affected the precision of salinity measurements. When water was in short supply, salinity was not measured.

The pH varies between 6.9 and 7.7. In general, pH drops over the upper 100 mcd and then rises over the lower 300 mcd (Fig. F39A). Difficulties were encountered when accurately measuring pH at high pH because the meter would not stabilize (perhaps from the incessant shaking due to ice breaking or temperature fluctuations). Fortunately, this difficulty does not significantly affect the determination of alkalinity, which mostly depends on the amount of HCl added.

Alkalinity increases from nominally 2.5 mM at the sediment/water interface to 3.1 mM at ~5 mcd (Fig. F38). Alkalinity then decreases to 1.6 mM at 50 mcd and remains low until 100 mcd (Fig. F39A). This low in alkalinity broadly corresponds to the low in pH. Between 100 and 200 mcd, alkalinity rises to 6.5 mM. Akalinity then increases to 8.0 mM at the base of the Cenozoic sediment package (399 mcd).

The upper 5 mcd has no ammonium (Fig. F38). Beneath this depth, NH₄⁺ steadily rises to ~500 µm at ~200 mcd and fluctuates irregularly between 500 and 1420 µM from 200 to 400 mcd (Fig. F39B). The sharp change in NH₄⁺ gradient at 5 mcd corresponds to the shallow high in alkalinity.

Shipboard discussion

Rhizone sampling of shallow sediment was successful. Although Rhizone samplers consistently gave less water than squeezed whole rounds for given 5–10 cm long intervals, they provide an easy means to collect pore waters with limited impact on the whole core. Moreover, Rhizone sampling enables the construction of high-resolution pore water chemistry profiles (Fig. F38) in the shallow high-porosity zone.

Three features in the shipboard pore water chemistry profiles are worth highlighting because they pertain to the main paleoceanographic goals of the cruise. These are shallow carbonate dissolution, deep sulfate reduction, and shallow ammonium oxidation.

Lithological and micropaleontological descriptions of sediment note a general absence of primary carbonate below 19 mcd. In particular, no carbonate tests of ostracodes, foraminifers, or nannofossils were found in the core catcher samples. It is possible that calcareous tests were never deposited in sediment below 19 mcd. However, this is also where pH and alkalinity drop below 7.4 and 2.5 mM, respectively. This means that pore waters below 19 mcd (and to at least 100 mcd) are more corrosive to carbonate tests than the overlying sediment or water column. Carbonate tests may have dissolved when they were buried in these corrosive pore waters.

The inflection in alkalinity at ~200 mcd suggests that a chemical reaction is adding substantial amounts of HCO₃^– at this depth without accompanying H⁺. The obvious candidate is sulfate reduction of organic carbon:

2CH₂O_(s) + SO₄^2– _(aq) → 2HCO₃^– _(aq) + H₂S_(aq).

Black firm clays were deposited during the Eocene below 200 mcd. The upper part of these firm clays may be driving sulfate reduction at present day. Interestingly, the firm clays host abundant pyrite and lie beneath dark bands in the sediment, which may be composed of other iron sulfide minerals. Organic matter in the black firm clays has probably reacted with dissolved SO₄^2– since they were buried, producing abundant H₂S and, ultimately, iron sulfide minerals.

A peak in alkalinity at ~200 mcd coincides with a change in gradient of NH₄⁺ to one that is steeper than above. The peak in alkalinity again suggests that some chemical reaction is producing HCO₃^– without accompanying H⁺. The NH₄⁺ profile further suggests that upward diffusing NH₄⁺ drives this reaction. This may be a zone where the two microbial reactions denitrification and anammox are coupled (e.g., Rysgaard et al., 2004). The ultimate source of NH₄⁺ may be the diagenesis of N-bearing organic carbon in the Eocene black firm clays.

Methane

Headspace CH₄ concentrations are low, ranging from 10 to 140 ppmv (Table T40). The highest value was found at ~340 mcd.

Low CH₄ concentrations are expected for the organic-lean sediments above 200 mcd but are somewhat surprising for the organic-rich Eocene black firm clays below 200 mcd. Degradation of organic carbon generally proceeds through a sequence of microbial reactions. This sequence includes sulfate reduction followed by methanogenesis; the latter is thought to occur at high rates once sulfate reduction has resulted in very low SO₄^2– concentrations. The Eocene sediments have high organic carbon content (TOC often exceeding 2 wt%). This is a sufficient quantity to drive complete sulfate reduction and methanogenesis, especially considering that the organic carbon is dominantly of marine origin and has a low thermal maturity (see below). However, surrounding pore waters have fairly high dissolved SO₄^2– concentrations (>7 mM) (Table T39) so that methanogenesis is not a dominant microbial pathway, at least at present day. The low CH₄ concentrations and high SO₄^2– concentrations below 200 mcd raise an intriguing question: why are microbes not readily consuming the abundant SO₄^2– and organic carbon?

Pore water chemistry

Calcium concentrations of pore waters show an unusual trend with respect to depth (Fig. F39A; Table T39). They are close to that of seawater (10 mM) in shallow sediment but then generally increase with depth to ~14 mM at ~200 mcd. Below this, pore water Ca varies between 13 and 15 mM. The inflection in the dissolved Ca profile at ~200 mcd suggests dissolution of Ca-bearing solids below this depth. The sediment chemistry results show anomalously high Ca/Al values from 200 to 400 mcd (Fig. F40), and the mineralogy results (Table T41) show anomalous amounts of calcite and gypsum from 200 to 400 mcd. Apparently, calcite, gypsum, or both are currently dissolving in the Eocene black firm clays.

Dissolved Fe concentrations are close to zero above 20 mcd and below 200 mcd (Fig. F39A). Between these depths, there is a broad peak centered at 80 mcd, where the Fe concentration exceeds 300 µM (Fig. F39A; Table T39). Before sulfate reduction, organic matter decomposition proceeds via iron oxide reduction. Presumably, small amounts of iron oxides are deposited on the seafloor and subsequently buried. Between ~20 and 200 mcd, microbes use these oxides to consume organic carbon, which releases Fe to pore waters. The dissolved Fe diffuses away from this zone, upward to where it reacts with oxygen to reprecipitate Fe oxides and downward to where it reacts with dissolved sulfide to precipitate pyrite. The color change of sediment at ~20 mcd may mark the loss of Fe oxides, and the high Fe, Fe/Al, and pyrite contents at ~200 mcd (Tables T41, T42) may indicate current precipitation of Fe sulfides.

Dissolved Mg concentrations are close to that of seawater (53 mM) in shallow sediment but steadily decrease with depth (Fig. F39A; Table T39). The lowest concentration (35 mM) is in the deepest pore water sample, taken at ~400 mcd. No obvious Mg-rich mineral (e.g., dolomite) preferentially occurs near the bottom of the Hole M0004A. A chemical gradient may extend across the hole, attesting to “basement” waters with low dissolved Mg.

The dissolved Mn profile shows two prominent peaks where concentrations exceed 200 µM (Fig. F39A; Table T39). The first peak is found between 1.5 and 80 mcd and is centered at 20 mcd; the second peak is found between 160 and 380 mcd and is centered between 200 and 300 mcd. Before iron oxide reduction, organic matter decomposition proceeds via manganese oxide reduction. Along with iron oxides, Mn oxides are probably deposited on the seafloor and subsequently buried. Certainly, the shallowest sediment examined (0.21 mcd) has a fairly high Mn content for marine sediment (2880 mg/kg) (Table T41). Between ~1.5 and 80 mcd, microbes use the Mn oxides to consume organic carbon, which releases Mn to pore waters. The dissolved Mn diffuses away from this zone, upward to where it reacts with dissolved oxygen to reprecipitate Mn oxides and downward to where it reacts with dissolved carbonate to precipitate rhodochrosite. The comparatively subtle peaks of solid-phase Mn content beginning at ~1.5 mcd may indicate precipitation of Mn oxides; the large peaks of solid-phase Mn content between ~28 and 105 mcd may indicate precipitation of rhodochrosite (Table T41). A rhodochrosite groundmass particle (~5 mm) was found at ~28 mcd (below). The deeper dissolved Mn peak is more difficult to explain in terms of the standard sequence of microbial reactions. It probably relates to the interval of high solid-phase Mn between 180 and 200 mcd, which may be composed of Mn oxides emplaced during a time of very low sedimentation.

Dissolved Na concentrations display an interesting profile with respect to depth (Fig. F39B; Table T39). In the shallowest samples, Na concentrations (~478 mM) are close to that of modern seawater (~470 mM). With depth, however, Na concentrations generally rise to a subsurface maximum of 489 mM centered at ~50 mcd and then decrease to 484 mM by 100 mcd. Sodium is often assumed to have a straight profile in shallow sediment because, with the exception of rare cases (e.g., near salt diapirs), there are no minerals that consume or release significant amounts of Na. It is possible that the profile reflects changes in bottom water salinity in the Arctic Ocean since the Last Glacial Maximum (e.g., McDuff, 1985; Adkins et al., 2002). This interpretation is not supported by the Cl^– measurements (Table T39), but we suspect that they have low precision.

Both S and SO₄^2– were measured on pore waters (Table T39). In general, the independent analyses give similar results, showing that S (as SO₄^2–) steadily drops from concentrations near that of seawater (28 mM) at the seafloor to 11 mM at 200 mcd (Fig. F39B). Below this depth, S (as SO₄^2–) slowly decreases to 8 mM. As expected from discussion of the shipboard alkalinity (above), dissolved SO₄^2– is being consumed below 200 mcd, presumably through sulfate reduction of organic carbon, a reaction that produces dissolved sulfide. Although we did not quantify dissolved sulfide, its presence is indicated in samples from 200 to 400 mcd. These samples produced a white precipitate when zinc acetate was added (Table T39).

Dissolved Si concentrations are fairly low (<300 µM) in the upper 180 m of sediment, fairly high (>1000 µM) from 200 to 300 mcd, and intermediate from 300 to 400 mcd. The overall profile suggests release of Si to pore waters between 200 and 300 mcd and its precipitation above and below these depths. We note the presence of abundant biogenic opal between 200 and 300 mcd, abundant zeolites at ~200 mcd, and abundant authigenic silica minerals below 300 mcd (Table T42). Presumably, biogenic opal in lithostratigraphic Unit 2 is slowly dissolving and releasing Si to surrounding lithostratigraphic units where it reprecipitates.

Dissolved Sr concentrations generally track those of dissolved Ca. Concentrations near the seafloor are close to that of seawater (90 µM). With depth, Sr concentrations steadily rise, reaching 160 µM by 260 mcd. After a drop, Sr concentrations rise again, reaching 187 µM at ~400 mcd. Possible explanations for this profile are the same as those for Ca.

Elemental composition of sediment

Background

Although all sediment samples were analyzed for 48 elements, useful results were obtained for only 25 elements. Fourteen elements (Al, Br, Ca, Cl, Fe, K, Mg, Mn, Rb, S, Si, Sr, Ti, and Zn) typically yielded analytical precision better than 5%. Results for most of these elements are presented as depth profiles (Fig. F40).

The remaining 11 elements (As, Ba, Co, Cr, Cu, Ga, Ni, P, Pb, Th, and V) often had low analytical precision (5%–30%). However, because the contents of these 11 elements varied significantly with depth, their downhole trends are meaningful. Depth profiles of Ba and Ni are presented as examples (Fig. F40). We discuss some of the profiles below.

Downcore profiles of “terrigenous elements”

Aluminosilicate minerals (e.g., clays and feldspars) typically host most of the Al, K, Rb, and Ti in marine sediment. With a few notable exceptions, these four elements show high contents over the upper 200 m, a major decrease between ~200 and 220 mcd, low contents between 220 and ~350 mcd, and high contents below ~350 mcd (Fig. F40). The overall profile is similar to the total peak area determined by XRD (Table T42), which appears to be dictated by the amount of silicate minerals. The exceptions to these trends are rocks, concretions, and nodules (Table T41).

The abundance of terrigenous material can be estimated by a normative calculation based on the concentration of Al in each sample and the concentration of Al in typical Post-Archean shale (PAAS), as follows:

%Terrigenous_sample = (Al_sample/Al_PAAS) × 100,

where the Al content of PAAS is ~100,000 ppm. Such normalization suggests that sediment between 0 and 200 mcd and below 350 mcd comprises between 80% and 100% terrigenous material (excepting concretions and nodules). Such high values agree with the predominance of silty clays in lithostratigraphic Subunits 1/1 through 1/5 and the lower part of Unit 3. Between ~220 and ~350 mcd, however, the terrigenous abundance decreases to as low as 7% and is usually not greater than 30%. These low values are consistent with abundant silica and organic matter in Units 2 and 3, which dilute the terrigenous component. Dilution of terrigenous material also occurs between 200 and 220 mcd. Here, however, the main dilutant is pyrite, evident from extreme Fe and S contents (below) and mineralogy (Table T42).

Changes in the type of terrigenous material are often reflected by variations in normalized abundances of terrigenous elements. As a preliminarily examination, we normalized K and Ti to Al. Profiles of Ti/Al and K/Al both show a prominent change with respect to depth (Fig. F41). Between 0 and ~220 mcd and below ~350 mcd, both elemental ratios are relatively low. By contrast, the intervening interval has relatively high Ti/Al and K/Al ratios. This suggests that the detrital material deposited in Unit 2 and the upper part of Unit 3 is distinctly different from overlying and underlying sediment. We note that this unit is characterized by a high abundance of K-feldspar, consistent with the elevated K content.

Downcore profile of silicon

The downcore profile of Si shows some similarities with those of terrigenous elements but also a major difference. Like the terrigenous elements, the contents are generally high (20–27 wt%) from 0 to 200 mcd and below 350 mcd. This is consistent with the high abundance of aluminosilicate minerals. There are also obvious drops in Si content between 200 and 220 mcd and in nodule samples, which are caused by dilution of authigenic minerals (e.g., pyrite). However, in contrast to the terrigenous elements, Si has its highest contents between 220 and 350 mcd. Here, Si typically exceeds 30 wt%. This reflects the high abundance of biogenic silica (~220 to ~313 mcd) or authigenic silica (~313 to ~350 mcd) in this interval.

Downcore profiles of iron and sulfur

For most of the sediment column, Fe contents are between 4 and 6 wt%. Somewhat analogous to Si, however, this downhole profile of Fe (Fig. F40) actually reflects major differences in sediment composition. Above 200 mcd and below 350 mcd (excluding pyrite nodules), most of the Fe probably resides in aluminosilicate minerals (e.g., clays). Between these depths, though, much of the Fe occurs in pyrite. This can be demonstrated using sediment chemistry results in two ways. First, S contents are close to zero above 200 mcd and below 350 mcd (excluding pyrite nodules); in the intervening interval, Fe and S are highly correlated, with S contents generally exceeding 4 wt%. Second, the Fe/Al ratio is low above 200 mcd and below 350 mcd; in the intervening interval, the Fe/Al ratio is high (Fig. F41). The confirmation comes from the mineralogy, which shows abundant pyrite between 200 and 350 mcd (Table T42) (see “Lithostratigraphy”).

As alluded to above, Subunit 1/6 (200–220 mcd) is greatly enriched in Fe, with contents exceeding 10 wt%. The S content of sediment and the mineralogy both indicate that this interval has an extremely high pyrite content. However, when Fe is normalized to Al, this depth interval does not stand out; in fact the Fe/Al ratio is lower in this interval than in Unit 2 below. This suggests that biogenic silica dilutes pyrite (and terrigenous components) in Unit 2. In other words, Subunit 1/6 and Unit 2 are distinct because Subunit 1/6 has a much lower amount of biogenic silica.

Downcore profiles of “evaporite elements”

Sediment contents of Cl and Br are strongly correlated (r² = 0.9) and exhibit clear downhole trends (Fig. F40). These elements generally have low contents (~5000 and 30 ppm, respectively) between 0 and 220 mcd and below ~310 mcd. Between these depth horizons and within lithostratigraphic Unit 2, sediment Cl and Br contents rise to >18,000 ppm and >100 ppm, respectively. The exceptions to these trends are the squeeze cake samples, which consistently have lower Cl and Br contents, usually by a factor of 2.

A simple explanation for the Cl and Br contents is that they are mostly added to sediment samples from evaporation of pore water during preparation for chemical analyses. In particular, if we assume a sediment porosity of 40% to 50% and normal seawater Cl concentrations, sediments should have nominally 5000–7000 ppm Cl when analyzed. The squeeze cake samples, therefore, would have lower Cl and Br contents because significant pore water has been extracted. At issue, however, are the high values in Unit 2. To explain these by evaporation from pore water, sediment porosity would have to exceed 70%.

We also note a slight increase in the Br/Cl ratio of sediment within Unit 2. Some of the Br in this interval may come from halogenated organic carbon.

Lithostratigraphic units

As discussed in “Lithostratigraphy,” the overall sediment column consists of several units defined on the basis of sediment composition and physical properties. For the most part, these units nicely coincide with major changes in sediment chemistry (and mineralogy, below).

However, there is an issue with the boundary between Unit 2 and Unit 3 at ~313 mcd. Except for the “evaporite elements,” most profiles of sediment chemistry show limited change at this depth; instead, as noted above, major changes in sediment chemistry are found at ~350 mcd (Fig. F40). The discrepancy might be explained by a change in silica phases. Sediment between ~220 and ~313 mcd (Unit 2) contains abundant biogenic opal, whereas sediment between ~313 and ~350 mcd (top of Unit 3) contains abundant authigenic silica, tentatively labeled crystoballite and tridymite (Table T41). Much of the biogenic opal appears to have been altered between ~313 and ~350 mcd so that this interval has similar overall chemistry to Unit 2 but low amounts of biogenic silica (an obvious change in sediment composition). Because the conversion of opal to authigenic silica decreases the porosity, there is less water in this interval and, hence, lower contents of evaporite elements. Therefore, if biogenic opal had not been altered in the upper 47 m of Unit 3, it would have likely been described as part of Unit 2 (see “Lithostratigraphy”).

Mineralogy of sediment

A total of 214 samples were analyzed for their mineralogical composition by XRD. Across this sample suite, peaks for 23 minerals were identified and quantified on XRD traces. The sum of all peak areas was calculated as a “total peak area” (Table T42). The total peak area shows major changes downhole. Between 0 and ~200 mcd and below ~350 mcd, total peak area is generally high. The exceptions are analyses of pyrite-rich sediments and pyrite nodules found between 380 and 402 mcd. By contrast, between ~200 and ~313 mcd, total peak area is relatively low. A transition of increasing peak area occurs from ~313 to ~350 mcd. The overall profile of total peak area probably relates to changes in lithology and sediment chemistry. Sediments dominated by terrigenous siliclastic minerals have a high total peak area, and sediments dominated by pyrite and biogenic silica have a low total peak area. The interval with abundant authigenic silica has moderate total peak area.

To account for major changes in lithology and bulk sediment mineralogy, peaks for each mineral were normalized to the total peak area (Table T42). There are several important downhole changes in these normalized mineral abundances, some which are discussed above and in “Lithostratigraphy.”

Organic geochemistry

Total carbon (TC), total sulfur (TS), and TOC contents were determined on 52 samples of sediment at the BCR (Table T43). TOC and Rock-Eval parameters were determined on a second set of samples at Alfred Wegner Institute (Germany) (Table T44).

Total carbon, sulfur, and organic carbon

Based on TC, TS, and TOC content, the sedimentary sequence can be divided into several intervals that generally correlate with the lithostratigraphic units or subunits described elsewhere (Fig. F42).

Samples from lithostratigraphic Subunits 1/1, 1/2, and 1/3 (0 to ~165 mcd) are generally characterized by low TC (0.2–0.4 wt%), low TS (0.0–0.30 wt%), and low TOC (0.2–0.4 wt%). The exceptions are samples characterized by high Mn or high P, which have higher TC and TOC. These may correspond to intervals with rhodochrosite or carbonate fluorapatite.

Samples from lithostratigraphic Subunit 1/4 (~165 to ~198 mcd) have very low TC (~0.1 wt%) and TOC (~0.1 wt%). The TS is also relatively low (<0.3%).

Samples from lithostratigraphic Subunit 1/5 (~193 to ~198 mcd) have moderate TOC (0.7–1 wt%). (No samples from this interval were analyzed for TC and TS, although the elemental chemistry indicates the samples would have moderate S content.)

Samples from lithostratigraphic Subunit 1/6 and Unit 2 (~198 to ~313 mcd) have high TC (>2.5 wt%), high TS (>4.9 wt%), and high TOC (>2.2 wt%). However, Subunit 1/6 is distinct from Unit 2 because it is characterized by extreme TS contents (>10 wt%). In fact, the absolute values reported in Table T43 may be inaccurate because the high S contents affect calibration of the instrument.

Samples from lithostratigraphic Unit 3 (~313 to ~424 mcd) have variable TC, TS, and TOC. Of the samples analyses, TC, TS, and TOC range between 1.8 and 3.5 wt%, 0.6 and 9.3 wt%, and 0.9 and 3.0 wt%, respectively.

Quality and maturity of organic matter

In immature sediments (TOC values >0.3 wt%), hydrogen index (HI) and oxygen index (OI) values of Rock-Eval pyrolysis are useful indicators for the characterization of the composition of the organic carbon fraction (i.e., to estimate the amount of terrigenous [higher plant] and aquatic [marine or freshwater] proportions) (e.g., Tissot and Welte, 1984; Stein, 1991). HI values <100 mg hydrocarbon (HC)/g C are typical of terrigenous organic matter (kerogen type III), whereas HI values of 300–800 mg HC/g C are typical of aquatic organic matter (kerogen types I and II).

HI values of organic matter show significant changes downcore (Figs. F43, F44; Table T44). Organic matter of lithostratigraphic Unit 1 is mainly characterized by HI values <100 mg HC/g C, indicating a terrigenous (higher plant) origin. In lithostratigraphic Unit 2 and the upper part of lithostratigraphic Unit 3 (~220 to ~350 mcd), however, HI values are generally between 150 and 350 mg HC/g C, suggesting significant amounts of aquatic (i.e., marine and/or freshwater algae type) organic matter. The lower part of Unit 3 (>350 mcd) is mostly characterized by low HI values of ~50 to 100 mg HC/g C, suggesting a dominance of terrigenous (higher plant) organic matter. The exceptions are Cores 302-M0004A-30X and 31X, from the middle part of lithostratigraphic Unit 3. These samples have HI values of ~350 mg HC/g C, again suggesting significant proportions of marine and/or freshwater algae-type organic matter.

T_max values also vary downhole (Table T44). T_max is >435°C in the upper part of the section (i.e., lithostratigraphic Subunits 1/2 and 1/3). This suggests the presence of refractive terrigenous organic matter. In Subunits 1/5 and 1/6 and Units 2 and 3, T_max values are <435°C and often <400°C, indicating immature, thermally unaltered organic matter.

Preliminary paleoenvironmental interpretation

The Pleistocene to Miocene interval (Subunits 1/1 to 1/4) is characterized by low TOC contents (<0.4 wt%) of terrigenous (higher plant) origin, as indicated by the low HI values. This is very similar to numerous late Quaternary organic carbon records from the central Arctic Ocean (e.g., Belicka et al., 2002; Stein et al., 1994, 2003, and further references therein). Low primary productivity due to sea ice coverage may have precluded the preservation and accumulation of significant amounts of marine organic matter.

The interval of Eocene sediment represented by Unit 2, on the other hand, has high TOC contents (1.5– >3 wt%). For much of this interval, elevated HI values of 150 to 350 mg HC/g C suggest significant amounts of preserved marine and/or freshwater algae material. Along with other sediment parameters (e.g., laminations, high biogenic silica, and pyrite), the organic carbon suggests an unusual depositional environment.

The Azolla event and the PETM are also apparent in organic carbon data. The Azolla event (Sample 302-M0004A-11X-CC) is characterized by the maximum TOC content of 4.2 wt%. Samples from the PETM (Samples 302-M0004A-30X-CC and 31X-CC) are characterized by the maximum HI values. Both intervals seem to be times of increased accumulation and preservation of aquatic (marine and/or freshwater algae type) organic matter.

Scanning electron microscopy

Thirteen sediment samples were examined using scanning electron microscope (Table T45). The primary purpose of these analyses was to characterize samples of “special interest.” Photomicrographs and descriptions of these samples are included in “Supplementary Material.”

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