Paleo-redox changes across the Paleocene-Eocene Thermal Maximum, Walvis
Ridge (ODP Sites 1262, 1263, and 1266): Evidence from Mn and U enrichment factors
Cecily O. J. Chun1,2, Margaret L. Delaney1, and James C. Zachos3
1 Ocean Sciences Department, Institute of Marine Sciences, University of
California, Santa Cruz, California, USA
2 Now at School of Ocean and Earth Science, University of Southampton,
National Oceanography Centre, Southampton, UK
3 Earth and Planetary Sciences, Institute of Marine Sciences, University of
California, Santa Cruz, California, USA
1 Chun et al., 2009PA001861R 7 June 2010 Abstract
An understanding of sediment redox conditions across the Paleocene-
Eocene Thermal Maximum (~55 Ma) is essential for evaluating changes in processes that control deep-sea oxygenation, as well as identifying the mechanisms responsible for driving the benthic foraminifera extinction. Sites cored on the flanks of Walvis Ridge (Ocean Drilling Program Leg 208, Sites 1262, 1266, and
1263) allow us to examine changes in bottom and pore-water redox conditions across a ~2 km depth-transect of deep-sea sediments of PETM age recovered from the South Atlantic. Here we present measurements of the concentrations of redox- sensitive trace metals manganese (Mn) and uranium (U) in bulk sediment as proxies for redox chemistry at the sediment-water interface and below. All three
Walvis Ridge sites exhibit bulk Mn enrichment factors (EF) ranging between 4 and 12 prior to the warming, values at crustal averages (Mn EF = 1) during the warming interval, and a return to pre-event values during the recovery period. U enrichment factors across the PETM remains at crustal averages (U EF = 1) at Site
1262 (deep) and Site 1266 (intermediate depth). U enrichment factors at Site 1263
(shallow), peaked at 5 immediately prior to the PETM and dropped to values near crustal averages during and after the event. All sites were lower in dissolved oxygen content during the PETM. Before and after the PETM, the deep and intermediate sites were oxygenated, while the shallow site was suboxic. Our geochemical results indicate that oxygen concentrations did indeed drop during the
2 Chun et al., 2009PA001861R 7 June 2010 PETM, but not sufficiently to cause massive extinction of benthic foraminifera.
3 Chun et al., 2009PA001861R 7 June 2010 1. Introduction
The Paleocene Eocene Thermal Maximum (PETM; ~55 Ma) is one of the most prominent warming events in Earth’s history and is related to pronounced environmental change. The ocean’s surface temperature increased rapidly by 5-6
°C at high latitudes and 4-5 °C in the tropics in < 40 kyr and then cooled to pre- event values within ~100 kyr [Sluijs et al., 2006; Zachos et al., 2003; Zachos et al.,
2006]. Benthic foraminifera experienced mass extinction throughout the global oceans [Kennett and Stott, 1991; Thomas, 2007]. Evidence for a global carbon cycle perturbation is the prevalent > 2.5 ‰ negative carbon isotope excursion
(CIE) found in both marine and terrestrial systems [Kennett and Stott, 1991; Koch et al., 1992]. In deep marine environments carbonate dissolution was pervasive
[Colosimo et al., 2006; Zachos et al., 2005]. Investigating the biogeochemical response to global warming during the Paleogene is important as a possible past analog to future warm periods.
A sudden large increase of carbon dioxide in the atmosphere is a likely contributor to this rapid climate change. Possible sources of carbon include release of methane from dissociation of gas hydrates in slope sediments [Dickens, 2000;
2003; Dickens et al., 1997; Dickens et al., 1995], flood basalt volcanism [Storey et al., 2007], and a variety of terrestrial reservoirs [Higgins and Schrag, 2006; Kurtz et al., 2003; Pancost et al., 2007]. The release of methane from gas hydrates and its rapid oxidation to carbon dioxide would have led to decreases in bottom-water
4 Chun et al., 2009PA001861R 7 June 2010 oxygen concentrations [Dickens, 2000]. Increased carbon dioxide in the atmosphere should also be associated with decreased dissolved oxygen concentrations in the oceans due to direct warming on gas exchange and increased ocean stratification [Matear and Hirst, 2003; Matear et al., 2000].
Oxygen deficiency in deep-waters could have lead to the widespread benthic foraminifera extinction observed at the onset of the PETM. However, rapid carbonate dissolution of the deep ocean, reduced primary productivity, or some combination of these could have also contributed to the benthic extinction event
[Thomas, 2007]. Ocean acidification and carbonate dissolution has the potential to stress benthic organisms [Ridgwell and Schmidt, 2010; Thomas et al., 1999;
Thomas, 1998]. Reduced primary productivity (i.e., a decrease in food supply to benthic foraminifera) should have resulted in increased bottom-water oxygen concentrations from decreased export particle flux resulting in lower oxygen utilization. Previous work has used assemblage data of benthic foraminifera across the PETM to suggest a decrease in dissolved oxygen concentrations [Kaiho et al.,
2006]. However, their approach has been shown to be problematic because their assemblage index correlates more strongly with particulate organic carbon rather than bottom-water oxygenation (see discussion in [Gooday, 2003] and references therein).
We evaluate the hypothesis that decreased oxygen concentrations across the PETM caused the benthic extinction event. In this study, we measure changes
5 Chun et al., 2009PA001861R 7 June 2010 in redox-sensitive element distributions in marine sediments in response to large perturbations in ocean chemistry. We present results from a depth transect of
South Atlantic sites to assess the vertical gradient in dissolved oxygen across the
PETM.
2. Background
2.1 Proxies
To assess qualitative changes in paleo-redox conditions, we analyzed bulk sediment concentrations of manganese (Mn) and uranium (U). These elements exhibit redox behavior between their solid and dissolved states. When measured in the same downcore sediment profile Mn and U record the position of the oxic/suboxic boundary (commonly referred to as ‘redoxcline’) [Mangini et al.,
2001]. However, because Mn is one of the first elements to be mobilized in the sequence of organic matter oxidation reactions in marine sediments, its interpretation must be carefully considered [Froelich et al., 1979; Tribovillard et al., 2006]. Both metals are incorporated within sub-surface sediments, reflecting the redox state of the pore-waters. The redox state of the pore-waters can differ from that of the overlying bottom-waters. Despite these limitations, Mn and U provide useful qualitative information about the depositional environment.
Mn is supplied to the oceans as oxide coatings and delivered by wind, rivers, or diffusion off shelf sediments [Calvert and Pedersen, 1993]. Dissolved
Mn2+ in seawater is thermodynamically unstable in the presence of oxygen and is
6 Chun et al., 2009PA001861R 7 June 2010 oxidized to insoluble Mn-oxides (MnO2). Within oxygenated sediments Mn- oxides deposited from the overlying water column are buried at the sediment-water interface, and dissolved Mn2+ concentrations are low [Burdige and Gieskes, 1983].
In oligotrophic and oxic open ocean environments Mn-oxides will be preserved as long as the sediment column remains oxic to a considerable depth.
The burial of Mn can be directly influenced by dissolved oxygen concentrations. Links between Mn accumulation and dissolved oxygen have been made to qualitatively reconstruct the oxygen minimum zone of past climates
[Dickens and Owen, 1994; Schenau et al., 2002]. During steady-state conditions the burial of Mn equals the input from its sources. When the oxygen minimum zone expands deeper in the water column, Mn-oxides previously deposited under oxygenated bottom waters become remobilized. This enhanced flux of Mn2+ is transported through diffusion and advection and ultimately buried as Mn-oxides under oxygenated bottom waters [Klinkhammer and Bender, 1980].
During the early diagenesis of organic matter within marine sediments, as oxygen concentrations decrease, Mn-oxides are used in microbially mediated reactions as the next available terminal electron acceptor. Mn is then remobilized from the solid, Mn-oxide, to a dissolved ,Mn2+, phase [Froelich et al., 1979]. This dissolved Mn2+ will remain in solution and diffuse upward through the sediment profile until it comes into contact with dissolved oxygen, at which point it will re- precipitate as Mn-oxides and form a sub-surface concentration peak [Burdige and
7 Chun et al., 2009PA001861R 7 June 2010 Gieskes, 1983]. The position of this newly precipitated Mn-oxide peak represents the redox boundary between oxic and suboxic pore waters, or ‘redoxcline’. Above this depth Mn is oxidized and below this depth Mn is reduced. The depth of the redoxcline can also be influenced by changes in the redox environments of the bottom-waters and pore-waters, rates of biogeochemical processes, and diffusion of dissolved elements.
Below the Mn-oxide subsurface peak dissolved Mn2+ concentrations increase such that an authigenic Mn-carbonate phase precipitates e.g. [Neumann et al., 2002; Schenau et al., 2002]. The presence of authigenic Mn-carbonates is a common feature of early diagenesis in marine sediments [Boyle, 1983; Gingele and Kasten, 1994; Pedersen and Price, 1982]. The carbonate is supplied either by the post-depositional degradation of organic matter, calcite dissolution, or anerobic oxidation of methane. Dissolved Mn2+ is supplied by the microbial reduction of
Mn-oxides within the sediments. Therefore, while Mn-carbonates precipitate in suboxic pore-waters, the sediment-water interface was likely well-oxygenated to preserve Mn-oxides at the sediment surface.
In oxygenated seawater and sediment pore-waters, uranium exists as the
4- soluble uranyl carbonate complex [UO2(CO3)3 ], but in suboxic sediments uranium precipitates as uraninite (UO2) [Calvert and Pedersen, 1993;
Klinkhammer and Palmer, 1991; Morford and Emerson, 1999]. Dissolved U is supplied to the sediments from the overlying bottom-waters. Under steady-state
8 Chun et al., 2009PA001861R 7 June 2010 conditions (i.e. constant sediment accumulation rates and bottom-water oxygen concentrations), authigenic U accumulates within suboxic sediments at a constant rate. During nonsteady-state conditions that shift the diagenetic redox horizons, peaks in authigenic U will form [Kasten et al., 2003]. Because U enrichment generally occurs below the sediment surface, the age of the sediments which hosts the enrichment are older than when the enrichment occurred. Therefore, the presence of authigenic U in a sediment section represents the most recent redox state of the host sediments as the original signal may have been overprinted.
Positive correlations between organic carbon flux and authigenic U enrichment in marine sediments have led to suggest the use of U as a proxy for particulate organic carbon [Chase et al., 2001; McManus et al., 2005]. Increases in organic carbon flux to the sediments will decrease the concentration of bottom- water oxygen via increased organic matter oxidation, shifting the position of redox boundaries within the sediments. As authigenic U precipitates in suboxic pore- waters, the dissolved U concentration decreases, allowing more dissolved U to diffuse into the pore-waters and continue authigenic U enrichment. Recent work has shown that in locations where bottom-water oxygen concentration is high, there is no relationship between authigenic U enrichment and organic carbon flux
[Morford et al., 2009]. Authigenic U can also be remobilized within sediments if the pore-waters become re-oxygenated, erasing previous enrichments [McManus et al., 2005].
9 Chun et al., 2009PA001861R 7 June 2010 2.2 Site Descriptions
Ocean Drilling Program (ODP) Leg 208 provided a depth-transect (~2.2 km water depth range) of deep-sea sediments recovered from the South Atlantic across the Paleocene-Eocene Boundary [Zachos et al., 2004; Zachos et al., 2005].
Our study uses three sites from this depth transect (Figure 1, Table 1). A geographically restricted depth transect allows us to assume that sediment inputs were similar at the three sites with time, with variations for processes that depend on water depth at given ages affecting the sediment record.
The main lithologic feature in PETM sections from Walvis Ridge is the rapid drop from > 80 weight-percent calcium carbonate to < 1 weight-percent calcium carbonate in ~20 kyr (Figures 2B, 3B, and 4B) [Zachos et al., 2005]. Prior to the PETM, all sites were above the calcite compensation depth (CCD) and calcium carbonate and clay sediments were slowly accumulating. At the onset of the event the CCD shoaled from below the deepest site (Site 1262 at 3600 m paleowater depth) to above the shallowest site (Site 1263 at 1500 m paleowater depth). During the rapid shoaling of the CCD, calcium carbonate that was previously deposited during the Paleocene was chemically eroded, as calcium carbonate in the ocean buffered the large release of carbon dioxide [Zeebe and
Zachos, 2007]. This “chemical erosion” event and lack of calcium carbonate deposition created the lower portion of the clay layer, characteristic of the PETM.
As a consequence, the calcium carbonate sediments that were deposited just prior
10 Chun et al., 2009PA001861R 7 June 2010 to the PETM are missing. At the onset of the PETM, all sites were below the CCD, and only clay accumulated. When the release of carbon dioxide to the oceans and atmosphere ceased, calcium carbonate dissolution slowed as the CCD moved deeper again and accumulation of calcium carbonate sediments resumed.
Comparing calcium carbonate weight percent across the PETM at different water depths provides us with time-slices while the CCD rapidly shoaled, then gradually recovered to deeper depths. This results in clay layers of varying thickness among the three sites; ~ 35 cm at the deepest site, ~20 cm at the intermediate site, and only ~5 cm at the shallowest site [Zachos et al., 2004]. The benthic foraminifera extinction event corresponds to the base of the clay layer at all three Walvis Ridge sites [Zachos et al., 2004]. Modeling studies have verified the development of thicker clay layers (representing the duration that a site was below the CCD) as a function of increasing water depth [Ridgwell, 2007; Zeebe and Zachos, 2007].
The absolute age and duration of the PETM remain somewhat uncertain
[Kuiper et al., 2008; Westerhold et al., 2009]. Nonetheless, relative ages and approximate durations can be constrained at various locations by a variety of methods including cyclostratigraphy, extraterrestrial helium flux, and carbon isotope stratigraphy [Farley and Eltgroth, 2003; Röhl et al., 2007; Westerhold et al., 2007]. Of particular utility is the shape of the CIE. High-resolution bulk carbon isotope records, which have been used for detailed correlation, show an initial
11 Chun et al., 2009PA001861R 7 June 2010 excursion in δ13C by as much as - 1.9 ‰ at Site 1263 (Figure 4A). The CIE at Site
1262 is - 1.4 ‰ (Figure 2A), coinciding with the base of the clay layer, then a gradual recovery to pre-excursion values of 2 ‰ at all sites (Figures 2A, 3A, and
4A) [Zachos et al., 2005]. During the PETM, bulk carbon isotopes are truncated by carbonate dissolution [McCarren et al., 2008; Zachos et al., 2007]. Therefore, the earliest part of the PETM and evidence for its original magnitude of the CIE are missing.
An age model for the PETM interval at Site 1263 has been derived by correlation to the Southern Ocean ODP Site 690 [Röhl et al., 2007; Zachos et al.,
2005]. The age model for Site 690 is based partly on cycle stratigraphy or through the counting of precession cycles. Site 1263 ages were transferred to the other
ODP Leg 208 sites using tie points identified through carbon isotopes, iron (Fe) concentration, biostratigraphic data, and bulk magnetic susceptibility records
[Zachos et al., 2005]. This method of site-to-site correlation and the use of cycle stratigraphy are preferable for comparing relative changes in sediment accumulation rates over brief intervals of time.
We divide the PETM at the Walvis Ridge sites into three time-intervals: (1) a ~50 kyr pre-event, including the calcium carbonate-rich section of the Paleogene prior to the Paleocene-Eocene boundary, (2) the “core-CIE”, the ~80 kyr duration that represents the onset, peak, and initial recovery phase of the CIE, and (3) the 12 Chun et al., 2009PA001861R 7 June 2010 recovery, ~ 80 - 170 kyr, return to near pre-event carbon isotope values. The 80 kyr duration of the core-CIE is identified by inflection point “E” originally used in the work of Zachos et al. [2005]. This point is where the carbon isotopes begin to return to pre-excursion values.
2.3 Samples and Analytical Procedures
Two hundred and eighty-four samples of ~1 cm3 volume were taken from
ODP 1262 Hole B, 1263 Holes A, C, D, and 1266 Holes B and C. The samples span from ~1-5 m below the PETM to 2-10 m above the PETM. Average sampling resolution was ~1 sample every 5 cm (~1 sample/6 kyr).
Bulk processing (a 50 mg split of all samples) followed a total-sediment acid-digestion method modified after the work of Murray and Leinen [1996]. All samples were freeze-dried, crushed, and sieved with a 150 μm polypropylene mesh. Samples were digested with 5 ml of concentrated nitric acid (16 N) and 1 ml of concentrated hydrofluoric acid (28.9N) overnight in 7 ml Teflon vials (Savillex
Corp. MN, USA). They were then microwaved at low power for 90 minutes and taken to dryness on a hot plate. Acids (1 ml each of 16 N HNO3, 12N HCl, and
16N HNO3 Trace Metal Grade) were sequentially added, with drying on the hot plate between each addition. Samples were then redissolved in 1ml 16 N HNO3,
0.5 ml 30% hydrogen peroxide (H2O2), 0.1 ml 28.9N HF, and 5 ml quartz distilled water.
13 Chun et al., 2009PA001861R 7 June 2010 Splits of 100 mg each were taken from 108 samples and subjected to a reductive cleaning procedure. The purpose of this step was to remove oxide and hydroxide phases. This procedure has been applied to distinguish the amount of
Mn in oxides and hydroxides from that in aluminosilicates and carbonates
[Dickens and Owen, 1993; Schenau et al., 2002]. The sample splits were added to
10 ml of a solution prepared by mixing 0.033 M sodium dithionite and complexation with 0.22 M sodium citrate and 1.0 M sodium bicarbonate solution
(pH = 7.6). The mixtures were continuously shaken for 6 hours, and the supernatant was decanted. We then added 10 ml of 1 M magnesium chloride
(MgCl2) (pH = 8.0), shaking for 2 hours, decanting the supernatant, and finally rinsing with 10 ml distilled water and shaking overnight. The remaining solid was then transferred to 7 ml Teflon vials and taken to dryness on a hot plate. Total sediment acid digestion (as described above) was then performed on the remaining residue.
Analyses for Mn, U, and Ti were performed using a Finnigan Element high-resolution inductively-coupled plasma-mass spectrometer (HR-ICP-MS). Al was measured on a Perkin-Elmer Optima 4300 DV inductively coupled plasma- optical emission spectrometer (ICP-OES). One internal standard composed of a homogenized mixture of Leg 208 samples was processed and analyzed with each batch of samples to evaluate precision. Mean concentrations ± 1 standard deviation
14 Chun et al., 2009PA001861R 7 June 2010 of the consistency standard were 152 ± 23 μmol g-1 for Al, 4.8 ± 0.8 μmol g-1 for
Ti, 5.0 ± 0.8 μmol g-1 for Mn, and 0.30 ± 0.2 nmol g-1 for U (Table 2).
3. Results
The results of bulk sample elemental analyses are presented in the auxiliary material (Tables S1-S6). Average bulk Mn concentrations are ~15 μmol g-1 at the deepest site, and ~6 μmol g-1 at the intermediate and shallowest sites. Average bulk U concentrations are ~1-2 nmol g-1 at all three sites. The concentrations of these elements increased within the clay layer. Mn concentrations in the clay layer average ~20 μmol g-1 at the deep and intermediate sites, and ~8 μmol g-1 at the shallowest site. Outside of the clay layer, average Mn concentrations are ~13 μmol g-1 at the deep site, and ~6 μmol g-1 at the intermediate and shallowest sites. U concentrations in the clay layer average between 3 - 5 nmol g-1 at all sites and decrease to between 1-2 nmol g-1 outside of the clay layer.
Bulk concentrations of these elements alone provide little information about the sedimentary process. Frequently, mass accumulation rates are calculated to express the rate of delivery to the sediments. However, precise age models are required as small changes in the linear sedimentation rates can cause major changes in mass accumulation rates. We choose to normalize our results, allowing us to make conclusions about relative changes downcore and among the three sites.
15 Chun et al., 2009PA001861R 7 June 2010 In order to distinguish between authigenic element enrichment factors (EF) reflecting changes in bottom-water conditions and changing detrital inputs with time, element concentrations are normalized to an assumed detrital index, Ti.
Trace-metal EF for Mn and U were calculated as EF =
(metal/Ti)sample/(metal/Ti)crust using the bulk crustal ratios Mn/Ti (mol/mol) = 0.156 and U/Ti (mmol/mol) = 0.061 [Rudnick and Gao, 2003].
Mn and U enrichment factors change across the PETM at all three sites, although the magnitude of change is not uniform with water depth. We describe our results using three time intervals to compare the period of carbon release and ocean warming to pre- and post- event conditions. Prior to the CIE, Mn enrichment factors at Sites 1262, 1266, and 1263 range between 2-8 (Figures 2C, 3C, and 4C, closed symbols). During the core-CIE, Mn enrichment factors drop to the crustal average (Mn EF = 1). During the recovery, Mn enrichment factors reach values slightly higher than pre-event. At the deep (Site 1262) and intermediate (Site 1266) sites, U enrichment factors remain at or near the crustal average (U EF < 2) throughout the sampled period (Figure 2D and 3D). Below the clay layer and in the pre-CIE interval at the shallowest (Site 1263) site, U enrichment factors peak to near 5, then drop rapidly to the crustal average at the PEB and remain low (U
EF < 2) for the remainder of the sampled period (Figure 4D).
We compare Mn enrichment factors from bulk sediment digested before and after a reductive cleaning procedure to identify Mn enrichments as Mn-oxides
16 Chun et al., 2009PA001861R 7 June 2010 or Mn-carbonates. Our reductive cleaning procedure targets easily reducible Mn- oxides such that when sediments with no U enrichment and Mn EF > 1 before reduction show Mn EF = 1 (crustal average) after reduction, the difference is inferred to be Mn-oxides that were preserved in well oxygenated bottom-waters.
When Mn enrichment remains > 1 in samples digested before and after reductive cleaning, these Mn enrichment factors represent Mn-carbonates (MnCO3).
Because Mn-carbonates are an early diagenetic feature that form within sub-oxic pore-waters, the overlying bottom-waters of these sediments could have been oxygenated. Therefore, our interpretation of the depositional environment containing Mn-carbonates is limited to relative comparisons of the other sites at the same time interval. Concentrations alone in the reductively cleaned samples cannot be directly compared to those in untreated samples because we did not re- weigh the samples after reduction; however, elemental ratios and enrichment factors can be directly compared.
Mn enrichment factors on the subset of samples that were reductively cleaned from the deep and intermediate sites are near crustal averages pre-CIE and during the recovery intervals (Mn EF < 2) (Figure 2C and 3C, open symbols).
Within the PETM interval, Mn enrichment factor values change very little before and after reductive cleaning at all sites. Reductive cleaning on the subset of samples from the shallowest site produced no change in Mn enrichment factors
(Figure 4C, open symbols).
17 Chun et al., 2009PA001861R 7 June 2010 4. Discussion
4.1 Early diagenetic processes and calcium carbonate dissolution
Similar Mn concentrations to the ones measured in this study before and after the PETM have been measured at lower resolution for different timescales in the Paleogene at Agulhas Ridge, Blake Nose, and Ceara Rise [Anderson and
Delaney, 2005a; b; Faul et al., 2003; Nilsen et al., 2003]. These locations represent typical open-ocean deep marine sections with generally low sedimentation rates of
0.5 -2.0 cm/ky. Low sedimentation rates at these locations suggest that the flux of organic matter reaching the sediment-water interface does not exceed the rate of remineralization, such that the bottom-waters remain well oxygenated. In these
‘steady-state’ situations we can assume Mn and U record the initial redox environment (bottom-water and/or pore-waters). Shifts in the background element distributions both within each site and along the depth transect would suggest
‘non-steady-state’ perturbations to the redox environment.
The elevated Mn and U concentrations across the clay layers likely reflect the loss of carbonate, which serves as a dilutant. During the start of the PETM, the
CCD shoaled several kms in the vicinity of Walvis Ridge [Zachos et al., 2005]. As such, the proportion of non-carbonate components increased.
Mn enrichment factors were at crustal averages during the CIE (Figures
2C, 3C, and 4C), indicating that the redox environment of the bottom-waters and/or pore-waters were not favorable to Mn-oxide preservation (Figures 5B and
18 Chun et al., 2009PA001861R 7 June 2010 6B). We interpret this lack of deposition to a shift in bottom-water oxygen levels that were lower than those before or after the event.
According to our computations, it is likely that most of the Mn at Site 1263 is present as Mn-carbonate. Before the CIE and during the recovery, the intervals where we presume Mn-carbonate formed, Site 1263 is ~90 weight percent CaCO3 and ~10 weight percent non-carbonate (Figure 4B). Using average Mn/Ca ratios in bulk sediment of 1-2 mmol/mol [Boyle, 1983; Pena et al., 2005], 10 g of bulk sediment contains 1.8 x 10-4 mol Mn (largest estimate, using a 2:1 ratio). We can then compare the average Mn excess concentration before and after the CIE. Mn excess is interpreted to be Mn not associated with terrigenous or detrital components. and calculated as Mn excess = Mntotal – [Tisample * (Mn/Ti)crust], using the bulk crustal Mn/Ti (mol/mol) = 0.156 [Rudnick and Gao, 2003].
The average Mn excess concentrations before the CIE and during the recovery at the shallow site are 4.12 and 6.33 μmol Mn respectively, equivalent to
4.12 x 10-5 and 6.33 x 10-5 mol Mn in 10 g bulk sediment compared to 1.8 x 10-4 mol Mn maximum estimated for the Mn-carbonates. Therefore, it is plausible that all of the ‘excess’ Mn measured at the shallowest site can be accommodated as
Mn-carbonates that were formed within suboxic pore-waters.
Massive calcium carbonate dissolution at the start of the PETM would have released dissolved Mn and U (originally associated with carbonates) to the overlying bottom-waters or pore-waters. During this period, both elements have
19 Chun et al., 2009PA001861R 7 June 2010 the potential to be redistributed according to diffusional gradients or become trapped within the host sediments below the clay layer. At the shallow site, dissolved Mn was incorporated within the host sediments as Mn-carbonates, while at the deep and intermediate sites pore-waters were more oxygenated and Mn re- precipitated as Mn-oxides (Figures 5A and 6A).
4.2 Post-depositional diagenesis
To characterize the depositional conditions as indicated by our redox- sensitive trace metals, we need to distinguish between the signals recorded as sediments accumulated, versus subsequent diagenetic overprints acquired through diffusion or other processes [Thomson et al., 1993]. Results from Mn enrichment factors at the shallowest site (Site 1263) differ from those of the deep (Site 1262) and intermediate (Site 1266) sites suggesting different bottom-water and pore- water oxygen concentrations during accumulation.
The shallowest site is also unique as it exhibits an elevated U enrichment factor immediately below the CIE and clay layer, covering ~80 cm that spans a time interval of ~35 kyr (Figure 4D). Based on the profile, with a sharp upper cutoff immediately below the clay layer and a gradual decline deeper in the sediment across ~80 cm (Figure 4D and 6B), we suggest this U enrichment anomaly is an overprint that reflects environmental conditions that existed at the start of the Eocene. Therefore, the measured U enrichment is a diagenetic feature that occurred in response to changing ocean chemistry during the PETM, but is
20 Chun et al., 2009PA001861R 7 June 2010 preserved in sediments that were deposited prior to the event (Figure 6B). Similar
U enrichments of this peak shape have been measured extensively on the Madeira abyssal plain (NE Atlantic) and are related to the downward migration of a redox front associated with the deposition of organic-rich turbidites [Colley et al., 1989;
Thomson et al., 1998a; Thomson et al., 1993; Thomson et al., 1998b]. Rapid shifts in bottom-water oxygen concentrations will influence the pore-waters and associated sediments previously deposited at that location.
In the case of the PETM, the enrichment of organic matter was not caused by turbidites. Instead, carbonate dissolution at the PETM would have increased organic matter in the clay layer in a manner somewhat analogous to emplacement of an organic-rich turbidite. The PETM at Site 1263 occurs with a sharp contact between grayish-brown ash-bearing clay above and light-gray nannofossil ooze below. High-resolution total organic carbon measurements have not been measured across the PETM. However, shipboard measurements of total organic carbon at this site ranged from 0. 00 – 0. 86 weight %, typical of well-oxidized pelagic sediments [Zachos et al., 2004]. More organic carbon in the clay layer rapidly depletes available oxygen as organic matter oxidizes and causes the host sediments below the redox front to become more suboxic, allowing for the formation of a U enrichment factor peak.
Benthic foraminiferal carbon isotope gradients suggest that Site 1263 was located in or near the oxygen minimum zone [McCarren et al., 2008]. The water
21 Chun et al., 2009PA001861R 7 June 2010 depth of this site during the Paleocene at 1500 m is near the oxygen minimum zone. In figure 7 we overlay the paleodepth locations of the Walvis Ridge sites on a dissolved oxygen profile for the modern North Pacific. If the carbonate ion gradient were reversed during the PETM relative to modern [Zeebe and Zachos,
2007] due to changes in deep ocean circulation [Bice and Marotzke, 2002], then we might predict that dissolved oxygen concentrations in the South Atlantic would look similar to North Pacific ones.
With Site 1263 located within a stratified oxygen minimum zone, this would have helped to preserve higher amounts of organic carbon in the sediments prior to and during the CIE. Shallow water depth and lower concentrations of dissolved oxygen both contribute to reduce sediment flux exposure to organic matter remineralization. The deeper sites, Site 1262 and Site 1266, would have been well below the oxygen minimum horizon.
4.3 Bottom water redox changes during the PETM
Our data suggest that at the onset of the PETM, all three sites became less oxygenated as the oxygen minimum zone expanded vertically. Mn concentrations were reduced to crustal averages because the delivery of Mn-oxides, which represent enrichments at the deep and intermediate sites, were reduced to Mn2+ either prior to burial or at the sediment-water interface [Dickens and Owen, 1994].
The shallowest site was already at a depth of low oxygen concentrations, and became suboxic during the PETM, allowing formation of Mn-carbonates and a
22 Chun et al., 2009PA001861R 7 June 2010 diagenetic overprint of U in the host sediments immediately below the CIE.
Similar mechanisms of expanded oxygen minimum zones to explain Mn redirection along vertical gradients have been used to describe patterns of ‘non- steady-state’ trace-metal enrichments [Dickens and Owen, 1993; 1994; Schenau et al., 2002].
We suggest an expanded oxygen minimum zone directly influenced the distribution of the trace metals rather than changes in the flux of organic carbon for three reasons: the distinct profile of U enrichment, U enrichment is not identical at the three sites, and U enrichment is not a robust proxy for organic carbon. We have shown that the structure of U enrichment factors at Site 1263 is similar in shape to authigenic U profiles formed as a consequence of emplacement of turbidites. These profiles are characteristics of post-depositional diagenetic imprints on the host sediments and most likely did not precipitate when the sediments were originally deposited. These explanations are contrary to studies that link authigenic U enrichments with organic carbon accumulation [Chase et al.,
2001; McManus et al., 2005].
The Walvis Ridge sites used in this study are in a geographically restricted area such that any changes in organic carbon flux to the sediments should be the same across all the sites. If organic carbon flux was primarily responsible for the trace element distribution, we would expect to measure the same distribution of the elements at all three sites. Specifically, we would expect uniform U enrichment
23 Chun et al., 2009PA001861R 7 June 2010 factors at all three sites. Because we only measure U enrichment factors at the shallowest site, the redox state of the bottom-waters is thought to have the greater influence on trace-element distribution rather than organic carbon flux. This provides further support that authigenic U is not a universal proxy for organic carbon flux [Morford et al., 2009].
4.4 Implications for the geologic record
During the PETM a major extinction of benthic foraminifera occurred throughout the ocean basins [Thomas, 2003]. The exact cause of the widespread extinction is unknown. One hypothesis attributes the benthic foraminifera extinction to oxygen deficiency of the deep-waters. We have evidence for a shift to suboxic conditions across the PETM, at all sites during the CIE, but not anoxia.
Modern epifaunal and infaunal benthic foraminifera have been found thriving in oxygen minimum zones [Gooday et al., 2000; Rathburn and Corliss, 1994].
Benthic foraminifera only cease to survive when bottom-water and pore-water oxygen concentrations become completely anoxic for long periods of time (on the order of decades) [Gooday, 2003].
Rapid ocean acidification and calcium carbonate dissolution have also been proposed as contributers to the benthic foraminifera extinction. We have already shown that calcite dissolution, in conjunction with an expanded oxygen minimum zone, provides a mechanism for release of dissolved Mn and U to the overlying bottom and pore-waters. Recent earth system models have shown that upon
24 Chun et al., 2009PA001861R 7 June 2010 releasing large quantities of carbon to the oceans and atmosphere during the
PETM, calcite saturation of the benthic environment becomes more undersaturated, and these conditions persist for several thousand years [Ridgwell and Schmidt, 2010]. Their study supports the idea that massive carbon release and the resulting ocean acidification contributed to unfavorable habitats for benthic foraminifera.
Increases in biogenic barium (Ba) accumulation as a proxy for export productivity in open-ocean marine sites have been proposed as a negative feedback to drawdown atmospheric CO2 during the PETM [Bains et al., 2000]. However, global increases in biogenic Ba (as barite crystals) poses a problem for the mass- balance of Ba [Dickens et al., 2003]. To accommodate the mass-balance problem,
Sr/Ba measurements in marine barite have been used to show that the seawater saturation state with respect to barite did not change across the PETM [Paytan et al., 2007]. Support for their model requires decreased burial of barite at continental margin settings to explain the increases measured in open-ocean sites.
Their model implies that open-ocean sites remained well oxygenated for the favorable preservation of marine barite. However, our study has shown that during the core CIE, pore-waters at all three sites on Walvis Ridge became suboxic, thus increasing the possibility of Ba remobilization during Mn-oxide dissolution [Dickens et al., 2003]. Detailed records of paleo-redox conditions during the PETM are important for validating the potential use of other
25 Chun et al., 2009PA001861R 7 June 2010 geochemical proxies. Existing records of barite preservation in open-ocean locations should be compared with their corresponding paleo-redox histories.
Our study is the first to use redox-sensitive trace metals to infer the relative oxygenation of the bottom and pore-waters across the PETM. We suggest that oxygen levels did not drop sufficiently to contribute to the mass extinction of benthic foraminifera [Gooday, 2003; Sen Gupta and Machain-Castillo, 1993].
Nevertheless, the bottom-water oxygen concentrations during the CIE were probably sufficiently low to affect metazoans and thus reduce bioturbation [Levin,
2003].
5. Summary
Variations in the oxidation state of the sediment-water interface and underlying pore-waters across the PETM are revealed in the elemental ratios of redox-sensitive trace metals in cores from a depth transect on Walvis Ridge. The comparison of Mn EF before and after a reductive cleaning procedure allows us to determine the presence of Mn-oxides or Mn-carbonates. Prior to the CIE, the deep
(Site 1262) and intermediate (Site 1266) sites were well oxygenated, while the shallow (Site 1263) site was situated within an oxygen minimum zone. During the core-CIE (~80 kyr), all three sites exhibited less-oxygenated bottom-waters.
During the recovery, the deep and intermediate sites returned to oxygenated bottom-waters while the shallow site remained in a suboxic environment. Our results support the hypothesis that oxygen concentrations declined, though not
26 Chun et al., 2009PA001861R 7 June 2010 sufficiently to cause widespread extinction of benthic foraminifera.
27 Chun et al., 2009PA001861R 7 June 2010 Acknowledgements
COJC thanks G. Dickens who originally suggested applying the reductive cleaning treatment prior to sediment digestion and for his detailed editorial handling. Sabine
Kasten and an anonymous reviewer are thanked for greatly improving the manuscript. We thank R. Franks and IMS for analytical support. J. Quartini, T.
Ivy, and E. Stokes provided help with sample preparation. This research used samples and/or data provided by the Integrated Ocean Drilling Program (IODP).
IODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of the Consortium for Ocean
Leadership. Funding for this research was provided by NSF grant EAR 0120727 to
JCZ and MLD. COJC was also supported by GAANN and ARCS fellowships through UCSC Ocean Sciences.
28 Chun et al., 2009PA001861R 7 June 2010
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37 Chun et al., 2009PA001861R 7 June 2010 Figure captions
Figure 1. Three-dimensional bathymetric map of Walvis Ridge highlighting the sites used in this study across a ~2.2 km water depth range (modified from Figure 3 in Zachos et al. [2004] figure courtesy of Ocean Drilling Program).
Figure 2. Geochemical tracers versus time relative to Paleocene-Eocene Boundary (PEB) for Walvis Ridge Ocean Drilling Program (ODP) Site 1262 (3600 m paleowater depth). See Table 2 for typical analytical errors. Gray shaded region indicates duration of core-carbon isotope excursion (CIE) ~ 80 kyr. Dashed vertical line is EF = 1 (no enrichment or depletion relative to presumed crustal source). (A) Bulk-sediment (δ13C) record [Zachos et al., 2005] ages after Röhl et al. [2007] (B) Weight % calcium carbonate content [Zachos et al., 2005] ages after Röhl et al. [2007] (C) Manganese enrichment factors (Mn EF) calculated from measured Mn concentrations and the crustal molar Mn/Ti of 0.156 [Rudnick and Gao, 2003]. Closed symbols indicate EF prior to reductive cleaning. Open symbols indicate EF post-reductive cleaning. (D) Uranium enrichment factors (U EF) calculated from measured U concentrations and the crustal molar U/Ti of 0.061 [Rudnick and Gao, 2003].
Figure 3. Geochemical tracers versus time relative to the Paleocene-Eocene Boundary (PEB) for Walvis Ridge ODP Site 1266 (2600 m paleowater depth). Dashed vertical line, gray shading, and molar ratios for EF calculations the same as in Fig. 2. (A) Bulk-sediment (δ13C) record [Zachos et al., 2005] ages after Röhl et al. [2007] (B) Weight % calcium carbonate content [Zachos et al., 2005] ages after Röhl et al. [2007] (C) Manganese enrichment factors (Mn EF). Closed symbols indicate EF prior to reductive cleaning. Open symbols indicate EF post- reductive cleaning. (D) Uranium enrichment factors (U EF).
Figure 4. Geochemical tracers versus time relative to the Paleocene-Eocene Boundary (PEB) for Walvis Ridge ODP Site 1263 (1500 m paleowater depth). Dashed vertical line, gray shading, and molar ratios for EF calculations the same as in Fig. 2. (A) Bulk-sediment (δ13C) record [Zachos et al., 2005] ages after Röhl et al. [2007] (B) Weight % calcium carbonate content [Zachos et al., 2005] ages after Röhl et al. [2007] (C) Manganese enrichment factors (Mn EF). Closed symbols indicate EF prior to reductive cleaning. Open symbols indicate EF post- reductive cleaning. (D) Uranium enrichment factors (U EF).
Figure 5. Schematic diagram of the development of Mn enrichment (Mn-rich oxyhydroxides) for Sites 1262 and 1266 across the PETM. Simplified solid phase 2+ Mn profiles and estimated O2 and Mn pore-water profiles (A) pre-event, (B) the 38 Chun et al., 2009PA001861R 7 June 2010 calcium carbonate dissolution phase (C) calcium carbonate recovery phase (D) post-event. Gray shaded region represents the ~80 kyr that the CCD is above Site 1262.
Figure 6. Schematic diagram of the development of Mn enrichment (Mn-rich oxyhydroxides) and U EF for Site 1263 across the PETM. Simplified 2+ concentration vs depth profiles for solid phase Mn, U, and estimated O2 and Mn pore-water profiles (A) pre-event, (B) the calcium carbonate dissolution phase (C) calcium carbonate recovery phase (D) post-event. Gray shaded region represents the ~80 kyr that the CCD is above Site 1262.
Figure 7. Dissolved oxygen concentrations (μmol/g) vs depth (m) for a Geochemical Ocean Section Study (GEOSECS) for a station in the North Pacific (33N, 139W) (data available at: http://ingrid.ldeo.columbia.edu/SOURCES/.GEOSECS/). Walvis Ridge site numbers are labeled at their approximate paleowater depth.
Table 1. Walvis Ridge Site Characteristics (ODP Leg 208) a Characteristic Site 1262 Site 1266 Site 1263 Modern water depth (meters) 4755 3798 2717 Paleowater depth (meters) b 3600 2600 1500 Latitude 27o11' S 28o33' S 28o32' S Longitude 1o35' E 2o21' E 02o47' E Depth range of samples used in this study (meters composite depth) 139.20-140.90 305.39-308.39 333.23-337.57 a Zachos, J.C., Kroon, D., Blum, P., et al. (2004) b Zachos et al. (2005)
39 Chun et al., 2009PA001861R 7 June 2010
Table 2. Analytical figures of merit Element Concentration (μmol/g sediment) Al Ti Mn Ua Detection Limitsb 18 0.34 0.93 0.50 Reproducibility (Consistency standard 152 ± 23 4.8 ± 0.8 5.0 ± 0.8 0.30 ± 0.2 replicates)c
a nmol/g sediment b Defined as three times the standard deviation of replicate measures of a blank of the same matrix as each digestion, and expressed in equivalent concentration for a typical size sediment sample c Reproducibility defined as ± 1 standard deviation of multiple replicates of a solid sample included in each analytical run (n = 14).
40 Chun et al., 2009PA001861R 7 June 2010