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Abstract Reconstructing the strength and depth boundary of oxygen minimum zones (OMZs) in the glacial ocean advances our understanding of how OMZs respond to climate changes. While many efforts have inferred better oxygenation of the glacial Arabian Sea OMZ from qualitative indices, oxygenation and vertical extent of the glacial OMZ is not well quantified. Here we present glacial‐Holocene oxygen reconstructions in a depth transect of Arabian Sea cores ranging from 600 to 3,650 m water depths. We estimate glacial oxygen concentrations using benthic foraminiferal surface porosity and benthic carbon isotope gradient reconstructions. Compared to the modern Arabian Sea, glacial oxygen concentrations were approximately 10–15 μmol/kg higher in the shallow OMZ (<1,000 m), and 5–80 μmol/kg lower at greater depths (1,500–3,650 m). Our results suggest that the OMZ in the glacial Arabian Sea was slightly better oxygenated but remained in the upper 1,000 m. We propose that the small increase in oxygenation of the Arabian Sea OMZ during the last glacial period was due to weaker upper ocean stratification induced by stronger winter monsoon winds coupled with an increase in oxygen solubility due to lower temperatures, counteracting the effects of more oxygen consumption resulting from higher primary productivity. Large‐scale changes in ocean circulation may have also contributed to better ventilation of the glacial Arabian Sea OMZ.
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Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean
Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growthof higher organisms. They also alter the marine nitrogen cycle, which isstrongly bound to the carbon cycle and climate. While higher organisms ingeneral start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygenconcentration below a threshold of about 20 µM (microbial hypoxia),whereas anoxic processes dominate the nitrogen cycle at oxygenconcentrations of < ∼ 0.05 µM (functionalanoxia). The Arabian Sea and the Bay of Bengal are home to approximately21 % of the total volume of ocean waters revealing microbial hypoxia.While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionallyanoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Eventhough there are a few isolated reports on the occurrence of anoxia prior to1960, anoxic events have so far not been reported from the open northernIndian Ocean (i.e., other than on shelves) during the last 60 years.Maintenance of functional anoxia in the Arabian Sea OMZ with oxygenconcentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses thesurface ocean circulation twice a year and turns vast areas of the ArabianSea from an oligotrophic oceanic desert into one of the most productiveregions of the oceans within a few weeks. Thus, the comparably lowvariability of oxygen concentration in the OMZ implies stable balancesbetween the physical oxygen supply and the biological oxygen consumption,which includes negative feedback mechanisms such as reducing oxygenconsumption at decreasing oxygen concentrations (e.g., reduced respiration).Lower biological oxygen consumption is also assumed to be responsible for aless intense OMZ in the Bay of Bengal. According to numerical model results,a decreasing physical oxygen supply via the inflow of water masses from thesouth intensified the Arabian Sea OMZ during the last 6000 years, whereas areduced oxygen supply via the inflow of Persian Gulf Water from the northintensifies the OMZ today in response to global warming. The first issupported by data derived from the sedimentary records, and the latterconcurs with observations of decreasing oxygen concentrations and aspreading of functional anoxia during the last decades in the Arabian Sea.In the Arabian Sea decreasing oxygen concentrations seem to have initiated aregime shift within the pelagic ecosystem structure, and this trend is alsoseen in benthic ecosystems. Consequences for biogeochemical cycles are asyet unknown, which, in addition to the poor representation of mesoscalefeatures in global Earth system models, reduces the reliability of estimatesof the future OMZ development in the northern Indian Ocean.
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- Award ID(s):
- 2019983
- PAR ID:
- 10219775
- Date Published:
- Journal Name:
- Biogeosciences
- Volume:
- 17
- Issue:
- 23
- ISSN:
- 1726-4189
- Page Range / eLocation ID:
- 6051 to 6080
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Reconstructing past oxygen fluctuations in oxygen minimum zones (OMZs) is crucial for understanding their response to climate change. Numerous studies suggest better oxygenation in the Arabian Sea OMZ during the Last Glacial Maximum (LGM) compared to the Holocene. However, bottom water oxygen (BWO) variability during the Penultimate Glacial Cycle (Marine Isotope Stage [MIS] 6 to MIS 5e, ∼140–115 ka B.P.) remains poorly constrained. This study reconstructs BWO variations during this period from sediment core TN041‐8JPC in the western Arabian Sea OMZ, utilizing proxies including benthic foraminiferal surface porosity, redox‐sensitive trace metal enrichment factors (e.g., UEF), and U/Ba ratios. Bottom water oxygen concentrations were 24.4 ± 5.9 μmol/kg during MIS 6 and 16.8 ± 6.5 μmol/kg during MIS 5e, with all proxies indicating higher BWO in MIS 6 than in MIS 5e. However, these proxies show different patterns within MIS 5e, indicating that UEFand U/Ba ratios may be limited to recording average BWO in glacial and interglacial (quasi)steady states. We propose that the intensified OMZ during MIS 5e, relative to MIS 6, was driven by higher productivity, temperature‐induced reductions in oxygen solubility, and reduced delivery of Southern‐sourced intermediate waters. In contrast, the intensified OMZ during the Holocene, compared to the LGM, was likely influenced by lower oxygen solubility, reduced Southern water delivery, and winter convective mixing rather than productivity. This study highlights a general trend of weaker OMZs in glacial than interglacial periods, though the mechanisms may not be identical, offering insights into OMZ dynamics under climate change in the past.more » « less
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Since 1980, atmospheric pollutants in South Asia and India have dramatically increased in response to industrialization and agricultural development, enhancing the atmospheric deposition of anthropogenic nitrogen in the northern Indian Ocean and potentially promoting primary productivity. Concurrently, ocean warming has increased stratification and limited the supply of nutrients supporting primary productivity. Here, we examine the biogeochemical consequences of increasing anthropogenic atmospheric nitrogen deposition and contrast them with the counteracting effect of warming, using a regional ocean biogeochemical model of the northern Indian Ocean forced with atmospheric nitrogen deposition derived from an Earth System Model. Our results suggest that the 60% recent increase in anthropogenic nitrogen deposition over the northern Indian Ocean provided external reactive nitrogen that only weakly enhanced primary production (+10 mg C.m–2.d–1.yr–1in regions of intense deposition) and secondary production (+4 mg C.m–2.d–1.yr–1). However, we find that locally this enhancement can significantly offset the declining trend in primary production over the last four decades in the central Arabian Sea and western Bay of Bengal, whose magnitude are up to -20 and -10 mg C.m–2.d–1.yr–1respectively.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/bg-17-6051-2020
