Iron limitation in the Western Interior Seaway during the Late Cretaceous OAE 3 and its role in phosphorus recycling and enhanced organic matter preservation

The sedimentary record of the Coniacian–Santonian Oceanic Anoxic Event 3 (OAE 3) in the North American Western Interior Seaway is characterized by a prolonged period of enhanced organic carbon (OC) burial. This study investigates the role of Fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black shale deposition within the Coniacian–Santonian-aged Niobrara Formation in the USGS #1 Portland core. Iron speciation results indicate the development of a reactive Fe limitation coeval with reduced bioturbation and increased organic matter preservation, suggesting that decreased sulfide buffering by reactive Fe may have promoted enhanced organic matter preservation at the onset of OAE 3. An Fe limitation would also provide a feedback mechanism to sustain elevated primary productivity through enhanced phosphorus recycling. Additionally our results demonstrate inconsistencies between Fe-based and trace metal redox reconstructions. Iron indices from the Portland core indicate a single stepwise change, whereas the trace metal redox proxies indicate fluctuating redox conditions during and after OAE 3. Using Fe speciation to reconstruct past redox conditions may be complicated by a number of factors, including Fe sequestration in diagenetic carbonate phases and efficient sedimentary pyrite formation in a system with limited Fe supply and high levels of export production

Isotopically Depleted Carbon in the Mid-Depth South Atlantic During the Last Deglaciation

The initial rise in atmospheric CO2 during the last deglaciation was likely driven by input of carbon from a 13C-depleted reservoir (Schmitt et al., 2012). Here we show that high resolution benthic foraminiferal records from the mid‐depth Brazil Margin display an abrupt drop in δ13C during Heinrich Stadial 1 (HS1) that is similar to but larger than in the atmosphere. Comparing the Brazil Margin results to published records from the North Atlantic, we are unable to account for the South Atlantic δ13C data with conservative mixing between northern and southern component watermasses. Rapid input of abyssal water from the Southeast Atlantic could account for deglacial δ13C anomalies at the Brazil Margin but it would require a reversal in deep water flow direction compared to today. A new mid-depth watermass may explain similar HS1 δ13C values in both the North and South Atlantic, but contrasting oxygen isotopic values between the two basins do not support the presence of a single dominant watermass at mid‐depths. Instead, it appears that δ13C behaved non-conservatively during the deglaciation, possibly reflecting the input of carbon from an isotopically depleted source.NSFPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98090/1/Tessin_Allyson_MS_2013.pdf1

Benthic phosphorus cycling within the Eurasian marginal sea ice zone

The Arctic Ocean region is currently undergoing dramatic changes, which will likely alter the nutrient cycles that underpin Arctic marine ecosystems. Phosphate is a key limiting nutrient for marine life but gaps in our understanding of the Arctic phosphorus (P) cycle persist. In this study, we investigate the benthic burial and recycling of phosphorus using sediments and pore waters from the Eurasian Arctic margin, including the Barents Sea slope and the Yermak Plateau. Our results highlight that P is generally lost from sediments with depth during organic matter respiration. On the Yermak Plateau, remobilization of P results in a diffusive flux of P to the seafloor of between 96 and 261 µmol m−2 yr−1. On the Barents Sea slope, diffusive fluxes of P are much larger (1736–2449 µmol m−2 yr−1), but these fluxes are into near-surface sediments rather than to the bottom waters. The difference in cycling on the Barents Sea slope is controlled by higher fluxes of fresh organic matter and active iron cycling. As changes in primary productivity, ocean circulation and glacial melt continue, benthic P cycling is likely to be altered with implications for P imported into the Arctic Ocean Basin

Southwest Atlantic water mass evolution during the last deglaciation

The rise in atmospheric CO₂ during Heinrich Stadial 1 (HS1; 14.5–17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean can liberate ¹³C-depleted carbon from the abyss, causing atmospheric CO₂ to increase and the δ¹³C of CO₂ to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained ¹³C-depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ¹⁸O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ¹⁸O signal suggests that correcting δ¹⁸O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ¹³C minima were most likely driven by an abrupt drop in δ¹³C of northern component water. Low δ¹³C at the Brazil Margin also coincided with an ~80‰ decrease in Δ¹⁴C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO₂ during HS1.This is the publisher’s final pdf. The published article is published by John Wiley & Sons Ltd. and copyrighted by American Geophysical Union. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9186/Keywords: carbon dioxide, deglaciation, South Atlantic, stable isotope

Millennial scale persistence of organic carbon bound to iron in Arctic marine sediments

Burial of organic material in marine sediments represents a dominant natural mechanism of long-term carbon sequestration globally, but critical aspects of this carbon sink remain unresolved. Investigation of surface sediments led to the proposition that on average 10-20% of sedimentary organic carbon is stabilised and physically protected against microbial degradation through binding to reactive metal (e.g. iron and manganese) oxides. Here we examine the long-term efficiency of this rusty carbon sink by analysing the chemical composition of sediments and pore waters from four locations in the Barents Sea. Our findings show that the carbon-iron coupling persists below the uppermost, oxygenated sediment layer over thousands of years. We further propose that authigenic coprecipitation is not the dominant factor of the carbon-iron bounding in these Arctic shelf sediments and that a substantial fraction of the organic carbon is already bound to reactive iron prior deposition on the seafloor

Southwest Atlantic water mass evolution during the last deglaciation

The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5–17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind‐driven upwelling in the Southern Ocean can liberate 13C‐depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained 13C‐depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid‐depth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.Key PointsDeep SW Atlantic was unlikely source of light carbon to atmosphere during HS1Mid‐depth isotopic anomalies due to change in northern component waterNorthern component water had robust influence in South Atlantic during HS1Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111970/1/palo20190.pd

Whole-mount in situ hybridization in the Rotifer Brachionus plicatilis representing a basal branch of lophotrochozoans

In order to broaden the comparative scope of evolutionary developmental biology and to refine our picture of animal macroevolution, it is necessary to establish new model organisms, especially from previously underrepresented groups, like the Lophotrochozoa. We have established the culture and protocols for molecular developmental biology in the rotifer species Brachionus plicatilis Müller (Rotifera, Monogononta). Rotifers are nonsegmented animals with enigmatic basal position within the lophotrochozoans and marked by several evolutionary novelties like the wheel organ (corona), the median eye, and the nonpaired posterior foot. The expression of Bp-Pax-6 is shown using whole-mount in situ hybridization. The inexpensive easy culture and experimental tractability of Brachionus as well as the range of interesting questions to which it holds the key make it a promising addition to the “zoo” of evo-devo model organisms

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data

Phytoplankton productivity and export sequester climatically significant quantities of atmospheric carbon dioxide as particulate organic carbon through a suite of processes termed the biological pump. Constraining how the biological pump operated in the past is important for understanding past atmospheric carbon dioxide concentrations and Earth\u27s climate history. However, reconstructing the history of the biological pump requires proxies. Due to their intimate association with biological processes, several bioactive trace metals and their isotopes are potential proxies for past phytoplankton productivity, including iron, zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver. Here, we review the oceanic distributions, driving processes, and depositional archives for these nine metals and their isotopes based on GEOTRACES-era datasets. We offer an assessment of the overall maturity of each isotope system to serve as a proxy for diagnosing aspects of past ocean productivity and identify priorities for future research. This assessment reveals that cadmium, barium, nickel, and chromium isotopes offer the most promise as tracers of paleoproductivity, whereas iron, zinc, copper, and molybdenum do not. Too little is known about silver to make a confident determination. Intriguingly, the trace metals that are least sensitive to productivity may be used to track other aspects of ocean chemistry, such as nutrient sources, particle scavenging, organic complexation, and ocean redox state. These complementary sensitivities suggest new opportunities for combining perspectives from multiple proxies that will ultimately enable painting a more complete picture of marine paleoproductivity, biogeochemical cycles, and Earth\u27s climate history

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data

Phytoplankton productivity and export sequester climatically significant quantities of atmospheric carbon dioxide as particulate organic carbon through a suite of processes termed the biological pump. Constraining how the biological pump operated in the past is important for understanding past atmospheric carbon dioxide concentrations and Earth\u27s climate history. However, reconstructing the history of the biological pump requires proxies. Due to their intimate association with biological processes, several bioactive trace metals and their isotopes are potential proxies for past phytoplankton productivity, including iron, zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver. Here, we review the oceanic distributions, driving processes, and depositional archives for these nine metals and their isotopes based on GEOTRACES-era datasets. We offer an assessment of the overall maturity of each isotope system to serve as a proxy for diagnosing aspects of past ocean productivity and identify priorities for future research. This assessment reveals that cadmium, barium, nickel, and chromium isotopes offer the most promise as tracers of paleoproductivity, whereas iron, zinc, copper, and molybdenum do not. Too little is known about silver to make a confident determination. Intriguingly, the trace metals that are least sensitive to productivity may be used to track other aspects of ocean chemistry, such as nutrient sources, particle scavenging, organic complexation, and ocean redox state. These complementary sensitivities suggest new opportunities for combining perspectives from multiple proxies that will ultimately enable painting a more complete picture of marine paleoproductivity, biogeochemical cycles, and Earth\u27s climate history