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doi:10.2204/iodp.proc.342.104.2014

Geochemistry

The geochemistry program during operations at Site U1403 included

• Analysis of interstitial gas compounds of headspace samples from Hole U1403A;

• Measurement of minor and trace element concentrations in interstitial water squeezed from whole-round samples from Hole U1403A; and

• Inorganic carbon, total carbon, and total nitrogen determinations of solid sediment samples from multiple holes.

Headspace gas samples

Twenty-five headspace gas samples were analyzed (Table T20) at a frequency of one sample per core in Hole U1403A as part of the routine environmental protection and safety monitoring program. The concentration of methane in most of the samples was close to detection limits (2.01 ppmv) and gradually increased to 16.19 ppmv with depth (maximum depth = 241.45 mbsf). The uppermost samples (Cores 342-U1403A-1H through 5H) have lower values than samples from below Core 6H, which also exhibit higher variability. Other hydrocarbon gases were not detected, which is consistent with the low organic carbon content of these sediments (see “Sediment geochemistry”).

Interstitial water geochemistry

Sixteen interstitial water samples were collected from Hole U1403A at a resolution of typically one sample per core (Table T21). Interstitial water samples were squeezed from 5 cm whole rounds taken on the catwalk. Most interstitial water samples were collected from the bottom of Section 6 or the bottom of Section 1 in a given core, depending on core disturbances visible through the core liner and proximity to intervals critical for stratigraphic correlation. Samples were not collected from key boundary intervals to preserve the stratigraphic integrity of the cores for future sampling. See “Lithostratigraphy” and “Stratigraphic correlation” for information on unit boundaries. Chemical constituents were determined according to the procedures outlined in “Geochemistry” in the “Methods” chapter (Norris et al., 2014b).

Salinity, pH, alkalinity, chloride, and sodium

The pH of interstitial water at Site U1403 ranges from 7.0 to 7.4, with higher values of 7.2–7.4 above 50 mbsf and fluctuating between 7.0 and 7.2 at greater depth. All values are lower than the average range of seawater pH, 7.7–8.1. Salinity ranges from 36‰ to 38‰, slightly higher than the average salinity of modern seawater (35‰), and shows a slight increasing downhole trend.

Alkalinity is relatively constant with depth, with most values between 3 and 4.5 mM. Exceptions are an anomalously high value of ~5.5 mM in Core 342-U1403A-8H and a low value of ~2.5 mM in Core 9H. All interstitial water samples have a higher alkalinity than average seawater (2.33 mM; International Association of Physical Sciences of the Ocean certified value).

Chloride concentrations were determined by manual titration and ion chromatography. The ion chromatography values are consistently offset to higher values compared to the manual titrations, which may have resulted from an aging ion chromatography column during analysis of Hole U1403A samples. Overall, chloride concentrations exhibit small fluctuations on a slightly increasing trend with depth. The sodium profile exhibits a decreasing trend with depth, with values of 490–525 mM above 170 mbsf and values of 480 mM or less below.

Carbonate crystallization and clay diagenesis as drivers of interstitial water profiles

Site U1403 downhole trends in potassium, calcium, and magnesium are consistent with those resulting from exchange and alteration with basaltic basement at depth (Gieskes, 1981), with potassium and magnesium decreasing from 13 to 6 mM and 59 to 37 mM, respectively, and calcium increasing from 11 to 49 mM with depth (Fig. F25).

An excursion in interstitial water potassium concentrations centered on the sample taken from Core 342-U1403A-14H coincides with the lithostratigraphic Unit III/IV boundary (see “Lithostratigraphy”). Low potassium levels in this interval are likely to reflect the sorption of potassium onto montmorillonite (Arthur, 1979). Pink blebs in Units III and IV were identified as authigenic montmorillonite by XRD (see “Lithostratigraphy”).

Strontium concentrations increase downhole from 80 µM in the shallowest sample to 200 µM. Both calcium and strontium concentrations in interstitial fluid increase steadily downhole in Hole U1403A, whereas the magnesium profile shows a linear decrease with depth. Although calcium and strontium can be influenced by carbonate dissolution and/or recrystallization during calcium carbonate diagenesis (e.g., Baker et al., 1982), there is no evidence for any significant deflection in the interstitial water trends at lithologic boundaries. Also, Sr/Ca exhibits a linear decreasing trend (except for a small positive fluctuation within lithostratigraphic Unit III), indicating that the conversion of biogenic carbonate to inorganic calcite is inactive. Hence, the calcium and strontium profiles are interpreted as resulting from diffusional processes. The regular, near-linear downhole profiles displayed by calcium, strontium, and plausibly magnesium indicate that these elements are affected by the same controlling factor at this site.

Sulfate, manganese, and iron

The sulfate interstitial water profile in Hole U1403A is characterized by a decreasing downhole trend from 30–35 mM in the shallowest samples to ~20 mM in the three lowermost samples.

Low alkalinity and high sulfate concentration indicate that sulfate reduction associated with the degradation of sedimentary organic matter (by the action of sulfate-reducing bacteria) has not been effective, presumably because of low organic content. This is consistent with the low methane concentration in headspace gas at this site. The manganese profile exhibits strong increases downhole from 0 to 196.5 µM. This trend is explained by general upward diffusion of dissolved manganese and its precipitation under the oxic conditions in the uppermost sediment. There is a small excursion in interstitial water sulfate and manganese concentrations near the lithostratigraphic Unit III/IV boundary (see “Lithostratigraphy”). Interstitial water concentrations of dissolved iron are low and invariant (0–1.5 µM).

Sediment geochemistry

Sediment samples

Samples for sediment geochemistry were taken at an interval of one per section in Hole U1403A, with the exception of Sections 342-U1403A-21X-2 through 21X-7 and 26X-2 through 26X-5, which were taken at 20 cm intervals to capture finer scale variations and potential lysocline shifts in sediment that capture the PETM and K/Pg boundary. Six sediment samples were taken from Sections 342-U1403B-26X-3 and 26X-4 to cover an interval of alternating light and dark bands in lithostratigraphic Subunit Va that was not observed in Hole U1403A.

Results

Carbonate determinations by coulometry were made for 248 samples from Site U1403 (Table T22). Calcium carbonate values range from 0.04 to 86 wt% (Fig. F26). Carbonate content in the uppermost two samples (from Sections 342-U1403A-1H-1 and 1H-2) are 35 and 25 wt%, respectively, and drop to almost zero between Sections 1H-3 and 13H-6. Below Core 342-U1403A-13H, near-zero carbonate contents were measured in three intervals, ~130, 180–200, and ~220 mbsf. In the lowermost ~20 mbsf in Hole U1403A, carbonate contents varied between 63 and 86 wt%, the highest carbonate values measured at Site U1403.

Elemental analysis of Hole U1403A sediment reveals that most of the total carbon in Hole U1403A sediment samples is present as calcium carbonate. All of the samples from Holes U1403A and U1403B were initially analyzed using National Institute of Standards and Technology 1646a Estuarine Sediment reference material (total carbon = 1.65 wt%) as calibration material. The calibration range for total carbonate is between ~0.03 and 0.33 mg (i.e., maximum calibration value of 2.2 wt% for a typical 15 mg sample; see Table T22). However, as was discovered during operations at Site U1403, the total carbon content of Hole U1403A sediment in high-carbonate samples (maximum value = 86 wt% CaCO₃) could be up to 4× higher than the maximum value used in the total carbon calibration. Thus, in many of the high-carbonate samples from Hole U1403A, the measured total carbon values are overestimated, requiring cross-calibration with a more suitable total carbon standard. We selected La Luna Shale with a total carbon of 11.52 wt%, corresponding to 2.3 mg in 20 mg, which was the highest calibration point.

Using the correction model described in “Geochemistry” in the “Methods” chapter (Norris et al., 2014b), we applied correction to samples with total carbon exceeding 2.2 wt% (Table T22). Corrected total carbon values range from <0.1 to ~12 wt%, which is very close to the range for inorganic carbon (<0.1–11 wt%), and show similar variation patterns to carbonate (Fig. F26).

Total organic carbon (TOC) is calculated by subtracting inorganic carbon from total carbon. Because of erroneous initial total carbon values, organic carbon values were also recalculated based on corrected total carbon contents (Table T22). Corrected TOC values vary between <0.1 and 3 wt% (Fig. F26). Excluding organic carbon values in high-carbonate samples, TOC values from low-carbonate samples are generally <0.5 wt%, with a few values up to 1.5 wt% in Cores 342-U1403A-4H and 21X that should be viewed with caution.

Total nitrogen is also reported (Fig. F26), although these values should also be used with caution. Anomalous total nitrogen data were removed because they were generally accompanied by anomalous total nitrogen values, as indicated by the standard. Most total nitrogen values vary between ~0 and 0.2 wt%.

Discussion of critical boundary intervals

Paleocene/Eocene Thermal Maximum

At depths below the Paleocene/Eocene boundary (~182 mbsf), Site U1403 lacks preserved carbonate, indicating it was deposited at a depth below the late Paleocene CCD. The position of the Paleocene/Eocene boundary in the core is somewhat uncertain because of a lack of biostratigraphic constraints for the upper Paleocene, but a prominent siliceous claystone layer at 182.2 mbsf in Hole U1403A (Fig. F27) may mark its location. Carbonate contents increase rapidly above this layer, quickly reaching ~30 wt% within a few tens of centimeters, with an abundance of calcareous nannofossils from Subzone NP9b (see “Biostratigraphy”). This abrupt increase could be the first direct evidence of a global carbonate oversaturation event, a CCD overshoot, during the recovery phase associated with the PETM. The carbon cycle model of Dickens et al. (1997) predicts such an overshoot, based on the increased rock weathering rates expected in response to elevated atmospheric CO₂ levels. In this model, the accelerated weathering rates increase the supply of alkalinity and dissolved inorganic carbon to the ocean, driving it into carbonate oversaturation and causing the CCD to deepen. Thus the large amounts of carbon released at the onset of the PETM ultimately results in greater preservation of calcium carbonate in sediment, as observed at this site. Carbonate oversaturation has previously been suggested by increased carbonate mass accumulation rates during the PETM recovery at ODP Sites 690 and 1266 (Kelly et al., 2010); however, the evidence of abrupt CCD deepening indicated by the results at Site U1403 places greater constraints on both the magnitude and duration of the excursion in the Atlantic CCD during the PETM recovery phase. These results therefore have the potential to better guide our understanding of carbon cycle perturbations during the PETM and the processes involved in restoring steady-state conditions. A similar increase in carbonate content occurs above the probable location of the ETM2 (~175 mbsf), suggesting that Site U1403 was situated at an appropriate depth to record evidence of CCD fluctuations through this time interval (Figs. F27).

Cretaceous/Paleogene boundary

The lithologic expression of the K/Pg boundary at Site U1403 occurs at the contact of lithostratigraphic Subunits Va and Vb (220.5 mbsf in Hole U1403A) (see “Biostratigraphy” and “Lithostratigraphy”). At Site U1403, calcium carbonate varies between 40 and 70 wt% across the K/Pg boundary and is absent in an interval a few meters above the boundary (214–216 mbsf) (Fig. F28).

Sediment above the K/Pg boundary (covering mainly nannofossil Zone NP4 and the Danian–Selandian transition; see “Biostratigraphy”) are predominantly biosiliceous calcareous chalks (<50 wt% CaCO₃), suggesting that Site U1403 was at or above the CCD during Late Cretaceous time. Foraminiferal tests and calcareous nannofossils are moderately to well preserved in this interval.

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