Effects of bottom water oxygen concentrations on mercury distribution and speciation in sediments below the oxygen minimum zone of the Arabian Sea

Abstract. In this study, we investigate phosphorus (P) and iron (Fe) cycling in sediments along a depth transect from within to well below the oxygen minimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-water and solid-phase analyses show that authigenic formation of calcium phosphate minerals (Ca-P) is largely restricted to where the OMZ intersects the seafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P (Corg/Porg) and to total reactive P (Corg/Preactive) in surface sediments indicate that the overall burial efficiency of P relative to Corg decreases under the low bottom water oxygen concentrations (BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sediments along the depth transect suggest that corresponding changes in Fe burial are limited. Sedimentary pyrite contents are low throughout the ~25 cm sediment cores at most stations, as commonly observed in the Arabian Sea OMZ. However, pyrite is an important sink for reactive Fe at one station in the OMZ. A reactive transport model (RTM) was applied to quantitatively investigate P and Fe diagenesis at an intermediate station at the lower boundary of the OMZ (bottom water O2: ~14 μmol L−1). The RTM results contrast with earlier findings in showing that Fe redox cycling can control authigenic apatite formation and P burial in Arabian Sea sediment. In addition, results suggest that a large fraction of the sedimentary Ca-P is not authigenic, but is instead deposited from the water column and buried. Dust is likely a major source of this Ca-P. Inclusion of the unreactive Ca-P pool in the Corg/P ratio leads to an overestimation of the burial efficiency of reactive P relative to Corg along the depth transect. Moreover, the unreactive Ca-P accounts for ~85% of total Ca-P burial. In general, our results reveal large differences in P and Fe chemistry between stations in the OMZ, indicating dynamic sedimentary conditions under these oxygen-depleted waters.

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Benthic Foraminifera from Middle to Late Pleistocene, coastal upwelling sediments of ODP Hole 686B, Pacific Ocean, off Peru

Journal of Micropalaeontology ◽

10.1144/jm.9.2.153 ◽

1991 ◽

Vol 9 (2) ◽

pp. 153-158 ◽

Cited By ~ 3

Author(s):

Kathryn A. Malmgren ◽

Brian M. Funnell

Keyword(s):

Late Pleistocene ◽

Surface Waters ◽

Benthic Foraminifera ◽

Bottom Water ◽

Coastal Upwelling ◽

Oxygen Minimum Zone ◽

Water Oxygen ◽

Minimum Zone ◽

Water Depths ◽

Oxygen Minimum

Abstract. Benthic Foraminifera from middle to late Pleistocene, (c. 600ka to 0ka), sediments of ODP Hole 686B, off Peru, show highest abundances and diversities during periods of cooler surface waters, (inferred from the Uk37 index), and enhanced upwelling, (inferred from the peridinacean/gonyaulacacean dinoflagellate cyst ratio). During the latest Pleistocene, (c. 160ka to 0ka), these periods are characterised by higher organic carbon contents in the bottom sediments, and occur during the odd-numbered, interglacial_18O stages. The benthic Foraminifera indicate deposition in 120 to 250 metres water depth for the earlier part of the record, (c. 600ka to c. 200ka), within the oxygen-minimum zone, with bottom water oxygen contents of <0.5 to 0.2 ml/l, (inferred from the dominance of Bolivinellina humilis). Deposition in water depths approaching those of the present day, (c. 450 metres), is indicated from c. 160ka onwards, with better oxygenated bottom water conditions, probably corresponding to the lower part of the oxygen-minimum zone.

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Sedimentary phosphorus and iron cycling in and below the oxygen minimum zone of the northern Arabian Sea

Biogeosciences Discussions ◽

10.5194/bgd-9-3829-2012 ◽

2012 ◽

Vol 9 (3) ◽

pp. 3829-3880 ◽

Cited By ~ 3

Author(s):

P. Kraal ◽

C. P. Slomp ◽

D. C. Reed ◽

G.-J. Reichart ◽

S. W. Poulton

Keyword(s):

Arabian Sea ◽

Bottom Water ◽

Surface Sediments ◽

Lower Boundary ◽

Oxygen Minimum Zone ◽

Apatite Formation ◽

Northern Arabian Sea ◽

Minimum Zone ◽

Oxygen Minimum ◽

Burial Efficiency

Abstract. In this study, we investigate phosphorus (P) and iron (Fe) cycling in sediments along a depth transect from within to well below the oxygen minimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-water and solid-phase analyses show that authigenic formation of calcium phosphate minerals (Ca-P) is largely restricted to where the OMZ intersects the seafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P (Corg/Porg) and to total reactive P (Corg/Preactive) in surface sediments indicate that the overall burial efficiency of P relative to Corg decreases under the low bottom water oxygen concentrations (BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sediments along the depth transect suggest that corresponding changes in Fe burial are limited. Sedimentary pyrite contents are low throughout the ~25-cm sediment cores at most stations, as commonly observed in the Arabian Sea OMZ. However, pyrite is an important sink for reactive Fe at one station in the OMZ. A reactive transport model (RTM) was applied to quantitatively investigate P and Fe diagenesis at an intermediate station at the lower boundary of the OMZ (bottom water O2: ~14 μ mol l−1). The RTM results contrast with earlier findings in showing that Fe redox cycling can control authigenic apatite formation and P burial in Arabian Sea sediment. In addition, results suggest that a large fraction of the sedimentary Ca-P is not authigenic, but is instead deposited from the water column and buried. Dust is likely a major source of this Ca-P. Inclusion of the unreactive Ca-P pool in the Corg/P ratio leads to an overestimation of the burial efficiency of reactive P relative to Corg along the depth transect. Moreover, the unreactive Ca-P accounts for ~85% of total Ca-P burial. In general, our results reveal large differences in P and Fe chemistry between stations in the OMZ, indicating dynamic sedimentary conditions under these oxygen-depleted waters.

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Temporally invariable bacterial community structure in the Arabian Sea oxygen minimum zone

Aquatic Microbial Ecology ◽

10.3354/ame01704 ◽

2014 ◽

Vol 73 (1) ◽

pp. 51-67 ◽

Cited By ~ 12

Author(s):

A Jain ◽

M Bandekar ◽

J Gomes ◽

D Shenoy ◽

RM Meena ◽

...

Keyword(s):

Community Structure ◽

Bacterial Community ◽

Bacterial Community Structure ◽

Arabian Sea ◽

Oxygen Minimum Zone ◽

Minimum Zone ◽

Oxygen Minimum

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Denitrification exceeds anammox as a nitrogen loss pathway in the Arabian Sea oxygen minimum zone

Deep Sea Research Part I Oceanographic Research Papers ◽

10.1016/j.dsr.2009.10.014 ◽

2010 ◽

Vol 57 (3) ◽

pp. 384-393 ◽

Cited By ~ 68

Author(s):

Silvia E. Bulow ◽

Jeremy J. Rich ◽

Hema S. Naik ◽

Anil K. Pratihary ◽

Bess B. Ward

Keyword(s):

Arabian Sea ◽

Nitrogen Loss ◽

Oxygen Minimum Zone ◽

Minimum Zone ◽

Oxygen Minimum

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Late Quaternary Variations in Monsoon Induced Upwelling and Oxygen Minimum Zone in the Eastern Arabian Sea: A Study Based on the Thecosome Pteropod Assemblage and Total Organic Carbon of Sediment

10.46427/gold2020.1707 ◽

2020 ◽

Author(s):

Jeet Majumder ◽

Anil Kumar Gupta ◽

Pankaj Kumar ◽

Kuppusamy Mohan ◽

Nirmal Biju

Keyword(s):

Organic Carbon ◽

Total Organic Carbon ◽

Arabian Sea ◽

Late Quaternary ◽

Oxygen Minimum Zone ◽

Eastern Arabian Sea ◽

Minimum Zone ◽

Oxygen Minimum

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Differences between coastal and open ocean distributions of N<sub>2</sub>O in the oxygen minimum zone off Peru

Biogeosciences Discussions ◽

10.5194/bgd-12-10167-2015 ◽

2015 ◽

Vol 12 (13) ◽

pp. 10167-10193 ◽

Cited By ~ 1

Author(s):

A. Kock ◽

D. L. Arévalo-Martínez ◽

C. R. Löscher ◽

H. W. Bange

Keyword(s):

Linear Relationship ◽

Coastal Upwelling ◽

Oxygen Minimum Zone ◽

Open Ocean ◽

Oxygen Utilization ◽

Double Peak Structure ◽

Peak Structure ◽

Minimum Zone ◽

Oxygen Minimum ◽

Oxygen Concentrations

Abstract. Depth profiles of nitrous oxide (N2O) were measured during six cruises to the upwelling area and oxygen minimum zone (OMZ) off Peru in 2009 and 2012/13, covering both the coastal shelf region and the adjacent open ocean. N2O profiles displayed a strong sensitivity towards oxygen concentrations. Open ocean profiles showed a transition from a broad maximum to a double-peak structure towards the centre of the OMZ where the oxygen minimum was more pronounced. Maximum N2O concentrations in the open ocean were about 80 nM. A linear relationship between ΔN2O and apparent oxygen utilization (AOU) could be found for all measurements within the upper oxycline, with a slope similar to studies in other oceanic regions. N2O profiles close to the shelf revealed a much higher variability, with N2O concentrations in the upper oxycline reaching up to several hundred nanomoles per liter at selected stations. Due to the extremely sharp oxygen gradients at the shelf, these maxima occurred in very shallow water depths of less than 50 m. In this area, a linear relationship between ΔN2O and AOU could not be observed. N2O concentrations above 100 nM were observed at oxygen concentrations ranging from close to saturation to suboxic conditions. Our results indicate that the coastal upwelling off Peru at the shelf causes conditions that lead to extreme N2O accumulation.

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Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

Biogeosciences ◽

10.5194/bg-17-6051-2020 ◽

2020 ◽

Vol 17 (23) ◽

pp. 6051-6080

Author(s):

Tim Rixen ◽

Greg Cowie ◽

Birgit Gaye ◽

Joaquim Goes ◽

Helga do Rosário Gomes ◽

...

Keyword(s):

Oxygen Consumption ◽

Indian Ocean ◽

Nitrogen Cycle ◽

Bay Of Bengal ◽

Arabian Sea ◽

Oxygen Supply ◽

Northern Indian Ocean ◽

Minimum Zone ◽

Oxygen Minimum ◽

Oxygen Concentrations

Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth of higher organisms. They also alter the marine nitrogen cycle, which is strongly bound to the carbon cycle and climate. While higher organisms in general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen concentration below a threshold of about 20 µM (microbial hypoxia), whereas anoxic processes dominate the nitrogen cycle at oxygen concentrations of < ∼ 0.05 µM (functional anoxia). The Arabian Sea and the Bay of Bengal are home to approximately 21 % of the total volume of ocean waters revealing microbial hypoxia. While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even though there are a few isolated reports on the occurrence of anoxia prior to 1960, anoxic events have so far not been reported from the open northern Indian Ocean (i.e., other than on shelves) during the last 60 years. Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the surface ocean circulation twice a year and turns vast areas of the Arabian Sea from an oligotrophic oceanic desert into one of the most productive regions of the oceans within a few weeks. Thus, the comparably low variability of oxygen concentration in the OMZ implies stable balances between the physical oxygen supply and the biological oxygen consumption, which includes negative feedback mechanisms such as reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration). Lower biological oxygen consumption is also assumed to be responsible for a less intense OMZ in the Bay of Bengal. According to numerical model results, a decreasing physical oxygen supply via the inflow of water masses from the south intensified the Arabian Sea OMZ during the last 6000 years, whereas a reduced oxygen supply via the inflow of Persian Gulf Water from the north intensifies the OMZ today in response to global warming. The first is supported by data derived from the sedimentary records, and the latter concurs with observations of decreasing oxygen concentrations and a spreading of functional anoxia during the last decades in the Arabian Sea. In the Arabian Sea decreasing oxygen concentrations seem to have initiated a regime shift within the pelagic ecosystem structure, and this trend is also seen in benthic ecosystems. Consequences for biogeochemical cycles are as yet unknown, which, in addition to the poor representation of mesoscale features in global Earth system models, reduces the reliability of estimates of the future OMZ development in the northern Indian Ocean.

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