More reducing bottom-water redox conditions during the Last Glacial Maximum in the southern Challenger Deep (Mariana Trench, western Pacific) driven by enhanced productivity (original) (raw)
Related papers
Chemical Geology, 2012
Keywords: Diatom mats Rare earth elements Cerium and europium anomalies Ethmodiscus rex West Pacific Last Glacial Maximum Rare earth elements (REE) along with selected major and trace elements were measured in laminated Ethmodiscus rex diatom mats and associated sediments (diatomaceous clay and pelagic clay) in two sediment cores collected from the Parece Vela Basin of the eastern Philippine Sea. REE distributions are characterized by strong enrichment of middle REE (MREE) and heavy REE (HREE) over light REE (LREE) and slight enrichment of MREE over HREE. This distribution pattern suggests that REE compositions of the laminated E. rex diatom mats were not the product of E. rex blooms, an inference also supported by the fact that Ti-normalized REE concentrations are not clearly different from those in the diatomaceous clay and pelagic clay. Comparison of REE patterns with the vertical distribution of authigenic phases in the sediment indicates that LREE were dominantly associated with detrital materials, whereas MREE and HREE were selectively enriched by Mn oxyhydroxides and authigenic phosphates. In the pelagic clay, a weak negative Ce anomaly cannot be linked to oxic depositional conditions but instead is attributed to the retention of a negative Ce anomaly associated with authigenic phosphates. In the laminated E. rex diatom mats, a weak positive Ce anomaly is consistent with depositional sulfidic anoxic conditions and is interpreted to represent enhanced scavenging of reduced aqueous Ce 3 + by barite in a strongly reducing environment. Moreover, variably negative to positive Ce anomalies in the diatomaceous clay may reflect the concurrent operation of the two processes controlling Ce accumulation in the laminated E. rex diatom mats and pelagic clay. The study cores show a ubiquitous positive Eu anomaly owing to the presence of volcanic materials. The smaller Eu anomaly in the laminated E. rex diatom mats and diatomaceous clay relative to that in the pelagic clay may be attributed to the release of reduced Eu 2 + from mineral phases to the sediment pore water under strongly reducing conditions. Previous studies have shown that the advection of Lower Circumpolar Water (LCPW) strengthened during the Last Glacial Maximum (LGM), which should have resulted in better ventilation of the deep eastern Philippine Sea. Evidence presented here for more reducing bottomwater conditions during the laminated E. rex diatom mats formation implicates elevated primary productivity and enhanced organic sinking fluxes as primary controls on redox changes in the eastern Philippine Sea during the LGM. Enhanced primary productivity and more reducing bottomwaters during the laminated E. rex diatom mats deposition resulted in a larger respired carbon pool in the deep eastern Philippine Sea, which may have been part of a larger-scale modulation of atmospheric CO 2 variation during Pleistocene glacial-interglacial cycles by deep-ocean carbon storage.
Chemical Geology, 2012
Paleoproductivity and paleoredox conditions were reconstructed in a sediment core in the Parece Vela Basin of the eastern Philippine Sea. The core consists of three units, from youngest to oldest: (1) laminated diatom mats (LDM) formed by Ethmodiscus rex during the Last Glacial Maximum (~18-28 kyr B.P.), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Elevated levels of export productivity during LDM deposition are indicated by high values for excess Ba, opal content, and TOC/Ti ratios. Estimated rates of organic carbon degradation (ca. 98%), opal mass accumulation (average 1322 g m −2 yr −1 ), and corrected organic carbon flux (average 248 g m −2 yr −1 ) are comparable to high-productivity regions of the modern ocean. The LDM is also characterized by moderate enrichment of redox-sensitive elements such as U, Mo, Cd, and Zn, highly 34 S-depleted pyrite sulfur isotopic compositions (indicating bacterial sulfate reduction in a sulfate-unlimited system), and C-S-Fe systematics reflecting limitation of pyrite formation by organic matter rather than reactive Fe availability. These features suggest mainly suboxic conditions in bottom waters but development of sulfidic-anoxic conditions at or close to the sedimentwater interface. Association of intensified anoxia with productivity maxima indicates that export production was a more important control on bottom water redox conditions than lateral ventilation. The DC and PC accumulated under oxic to suboxic conditions. Our observations suggest that redox environments during deposition of laminated marine sediments are more complicated and varied than previously thought, and, thus, the use of sediment lamination as an indicator of anoxic bottom water conditions must be approached cautiously.
Chemical Geology, 2012
Paleoproductivity and paleoredox conditions were reconstructed in a sediment core in the Parece Vela Basin of the eastern Philippine Sea. The core consists of three units, from youngest to oldest: (1) laminated diatom mats (LDM) formed by Ethmodiscus rex during the Last Glacial Maximum (~18-28 kyr B.P.), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Elevated levels of export productivity during LDM deposition are indicated by high values for excess Ba, opal content, and TOC/Ti ratios. Estimated rates of organic carbon degradation (ca. 98%), opal mass accumulation (average 1322 g m −2 yr −1), and corrected organic carbon flux (average 248 g m −2 yr −1) are comparable to high-productivity regions of the modern ocean. The LDM is also characterized by moderate enrichment of redox-sensitive elements such as U, Mo, Cd, and Zn, highly 34 S-depleted pyrite sulfur isotopic compositions (indicating bacterial sulfate reduction in a sulfate-unlimited system), and C-S-Fe systematics reflecting limitation of pyrite formation by organic matter rather than reactive Fe availability. These features suggest mainly suboxic conditions in bottom waters but development of sulfidic-anoxic conditions at or close to the sedimentwater interface. Association of intensified anoxia with productivity maxima indicates that export production was a more important control on bottom water redox conditions than lateral ventilation. The DC and PC accumulated under oxic to suboxic conditions. Our observations suggest that redox environments during deposition of laminated marine sediments are more complicated and varied than previously thought, and, thus, the use of sediment lamination as an indicator of anoxic bottom water conditions must be approached cautiously.
Global and Planetary Change, 2013
Giant marine diatoms, blooming or aggregating within deep chlorophyll maxima under stratified conditions, can generate substantial production and a large export flux of organic carbon from surface waters. However, their role in regulating glacial-interglacial variation in atmospheric pCO 2 remains unclear. Here, we report the organic carbon isotopic compositions of Ethmodiscus rex diatoms (δ 13 C E. rex ) and bulk sediments (δ 13 C org ) from a sediment core in the eastern Philippine Sea dated to~19.5-31.0 kyr B.P. and consisting of (from youngest to oldest) (1) laminated E. rex diatom mats (LDM), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Our results suggest that δ 13 C E. rex provides a better record of palaeoceanographic processes during LDM and DC deposition than δ 13 C org because of reduced differential vital effects in near-monospecific E. rex fractions. We used the isotopic composition of the coarse E. rex fraction (δ 13 C E. rex (>154 μm) ) to calculate the CO 2 partial pressure of eastern Philippine Sea surface waters (pCO 2-sw ) during the Last Glacial Maximum (LGM). Our pCO 2-sw records suggest that the eastern Philippine Sea switched from being a strong CO 2 source during DC deposition to a weak CO 2 sink during LDM deposition. The role of the eastern Philippine Sea as a CO 2 sink during the LGM was promoted by elevated primary production and, to a lesser extent, intensified water-column stratification. These observations highlight the potential role of giant marine diatoms in the sequestration of atmospheric CO 2 during the LGM and, hence, support changes in biogenic silica fluxes as a potential cause of lower glacial atmospheric CO 2 . Our findings are consistent with an eolian source of silica, as proposed by the 'silica hypothesis' and the 'silicon-induced alkalinity pump hypothesis' but not by the 'silicic acid leakage hypothesis.'
Rare earth element geochemistry of diatom mats from tropical West Pacific (2012, Chemical Geology)
Chemical Geology, 2012
Keywords: Diatom mats Rare earth elements Cerium and europium anomalies Ethmodiscus rex West Pacific Last Glacial Maximum Rare earth elements (REE) along with selected major and trace elements were measured in laminated Ethmodiscus rex diatom mats and associated sediments (diatomaceous clay and pelagic clay) in two sediment cores collected from the Parece Vela Basin of the eastern Philippine Sea. REE distributions are characterized by strong enrichment of middle REE (MREE) and heavy REE (HREE) over light REE (LREE) and slight enrichment of MREE over HREE. This distribution pattern suggests that REE compositions of the laminated E. rex diatom mats were not the product of E. rex blooms, an inference also supported by the fact that Ti-normalized REE concentrations are not clearly different from those in the diatomaceous clay and pelagic clay. Comparison of REE patterns with the vertical distribution of authigenic phases in the sediment indicates that LREE were dominantly associated with detrital materials, whereas MREE and HREE were selectively enriched by Mn oxyhydroxides and authigenic phosphates. In the pelagic clay, a weak negative Ce anomaly cannot be linked to oxic depositional conditions but instead is attributed to the retention of a negative Ce anomaly associated with authigenic phosphates. In the laminated E. rex diatom mats, a weak positive Ce anomaly is consistent with depositional sulfidic anoxic conditions and is interpreted to represent enhanced scavenging of reduced aqueous Ce 3 + by barite in a strongly reducing environment. Moreover, variably negative to positive Ce anomalies in the diatomaceous clay may reflect the concurrent operation of the two processes controlling Ce accumulation in the laminated E. rex diatom mats and pelagic clay. The study cores show a ubiquitous positive Eu anomaly owing to the presence of volcanic materials. The smaller Eu anomaly in the laminated E. rex diatom mats and diatomaceous clay relative to that in the pelagic clay may be attributed to the release of reduced Eu 2 + from mineral phases to the sediment pore water under strongly reducing conditions. Previous studies have shown that the advection of Lower Circumpolar Water (LCPW) strengthened during the Last Glacial Maximum (LGM), which should have resulted in better ventilation of the deep eastern Philippine Sea. Evidence presented here for more reducing bottomwater conditions during the laminated E. rex diatom mats formation implicates elevated primary productivity and enhanced organic sinking fluxes as primary controls on redox changes in the eastern Philippine Sea during the LGM. Enhanced primary productivity and more reducing bottomwaters during the laminated E. rex diatom mats deposition resulted in a larger respired carbon pool in the deep eastern Philippine Sea, which may have been part of a larger-scale modulation of atmospheric CO 2 variation during Pleistocene glacial-interglacial cycles by deep-ocean carbon storage.
Proterozoic ocean redox and biogeochemical stasis
Proceedings of the National Academy of Sciences, 2013
The partial pressure of oxygen in Earth's atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5-0.543 Ga). However, the trajectory and mechanisms of Earth's oxygenation are still poorly constrained, and little is known regarding attendant changes in ocean ventilation and seafloor redox. We have a particularly poor understanding of ocean chemistry during the mid-Proterozoic (∼1.8-0.8 Ga). Given the coupling between redoxsensitive trace element cycles and planktonic productivity, various models for mid-Proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients, with potential ecological constraints on emerging eukaryotic life. Here, we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates. We find that the metal enrichment record implies a Proterozoic deep ocean characterized by pervasive anoxia relative to the Phanerozoic (at least ∼30-40% of modern seafloor area) but a relatively small extent of euxinic (anoxic and sulfidic) seafloor (less than ∼1-10% of modern seafloor area). Our model suggests that the oceanic Mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of Mo and chromium enrichments through time is consistent with the possibility of a Mo-N colimited marine biosphere during many periods of Earth's history. paleoceanography | geobiology
Ediacaran ocean redox evolution
2015
The relative role that environmental versus intrinsic biological factors played in shaping the history of life on Earth is a fundamental but unanswered question in the natural sciences. Most famously, it has been heavily debated if the emergence and diversification of early animals during the Ediacaran Period (ca. 635-541 Ma) was tied to a rise in atmospheric-oceanic oxygen levels. Temporally discontinuous geochemical data and patchy fossil record in literature are inadequate to chart the Ediacaran redox history and its causal relationship with the biotic evolution. My PhD study aims for a multiproxy geochemical record within a well-established stratigraphic framework of the Ediacaran (ca. 635-541 Ma) succession in South China. This succession is particularly suited for obtaining a high-resolution redox record based on redox-sensitive trace elements (RSEs) and pyrite sulfur isotopes due to the availability of organic-rich black shales. Modern-level RSE enrichments in black shales immediately overlying the Marinoan-age glacial diamictites reveal, for the first time, a pervasive ocean oxygenation event in the aftermath of the Earth's extreme cold. This oxygenation event may have triggered the first appearance of primitive animals in Earth history. Following this oxygenation, RSE concentrations returned back to crustal and near-crustal values until viii TABLE OF CONTENTS
Paleoceanography, 2002
The Matuyama Diatom Maximum (MDM) is a time of peak opal accumulation from 2.6 to $2.0 Ma within the Benguela Current upwelling system that was initiated by increased influence of Southern Ocean water on the eastern South Atlantic. We measured opal, total organic carbon (TOC), and CaCO 3 fluxes and C and N stable isotopes in sediments deposited from 2.4 to 1.95 Ma at Sites 1082 and 1084 to explore the biogeochemical dynamics within the Benguela region. The infusion of Southern Ocean water delivered dissolved nutrients and Southern Ocean flora and fauna, resulting in local opal accumulation increasing up to 8 g/cm 2 ky and the production of diatom mats. Some d 15 N measurements of diatom-bound organic matter indicate that the mats grew within the Benguela region. The bulk sediment d 15 N records are taken to reflect changes in the d 15 N of nitrate in the incoming water, where lower values at 2.4 Ma reflect less nitrate utilization in the Antarctic. A long-term increase in relative nitrate uptake in the Southern Ocean is evidenced by the gradual increase in d 15 N toward 1.9 Ma.