Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

Abstract. At present, the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 and 1200 m. Active denitrification in the upper part of the OMZ is recorded by enhanced δ15N values in the sediments. Sediment cores show a δ15N increase during the middle and late Holocene, which is contrary to the trend in the other two regions of water column denitrification in the eastern tropical North and South Pacific. We calculated composite sea surface temperature (SST) and δ15N ratios in time slices of 1000 years of the last 25 kyr to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Low δ15N values of 4–7 ‰ during the last glacial maximum (LGM) and stadials (Younger Dryas and Heinrich events) suggest that denitrification was inactive or weak during Pleistocene cold phases, while warm interstadials (ISs) had elevated δ15N. Fast changes in upwelling intensities and OMZ ventilation from the Antarctic were responsible for these strong millennial-scale variations during the glacial. During the entire Holocene δ15N values > 6 ‰ indicate a relatively stable OMZ with enhanced denitrification. The OMZ develops parallel to the strengthening of the SW monsoon and monsoonal upwelling after the LGM. Despite the relatively stable climatic conditions of the Holocene, the δ15N records show regionally different trends in the Arabian Sea. In the upwelling areas in the western part of the basin, δ15N values are lower during the mid-Holocene (4.2–8.2 ka BP) compared to the late Holocene (< 4.2 ka BP) due to stronger ventilation of the OMZ during the period of the most intense southwest monsoonal upwelling. In contrast, δ15N values in the northern and eastern Arabian Sea rose during the last 8 kyr. The displacement of the core of the OMZ from the region of maximum productivity in the western Arabian Sea to its present position in the northeast was established during the middle and late Holocene. This was probably caused by (i) reduced ventilation due to a longer residence time of OMZ waters and (ii) augmented by rising oxygen consumption due to enhanced northeast-monsoon-driven biological productivity. This concurs with the results of the Kiel Climate Model, which show an increase in OMZ volume during the last 9 kyr related to the increasing age of the OMZ water mass.

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Glacial-Interglacial changes and Holocene variations in Arabian Sea denitrification

10.5194/bg-2017-256 ◽

2017 ◽

Cited By ~ 2

Author(s):

Birgit Gaye ◽

Anna Böll ◽

Joachim Segschneider ◽

Nicole Burdanowitz ◽

Kay-Christian Emeis ◽

...

Keyword(s):

Arabian Sea ◽

Late Holocene ◽

Asian Monsoon ◽

Sediment Cores ◽

Southwest Monsoon ◽

Northeast Monsoon ◽

Biological Productivity ◽

Minimum Zone ◽

Monsoon System ◽

Oxygen Minimum

Abstract. At present the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 m and 1200 m. Active denitrification in this OMZ is recorded by enhanced δ15N values in the sediments. Sediment cores show a δ15N increase from early to late Holocene which is contrary to the trend in other regions of water column denitrification. We calculated composite sea surface temperature (SST) and δ15N in time slices of 1000 years of the last 25 ka to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Pleistocene stadial δ15N values of 4–6 ‰ suggest that denitrification was inactive or weak. During interstadials (IS) and the entire Holocene, δ15N values of > 7 ‰ indicate enhanced denitrification and a stronger OMZ. This coincides with active monsoonal upwelling along the western margins of the basin as indicated by low SST. Stronger ventilation of the OMZ in the early to mid-Holocene period during the most intense southwest monsoon and vigorous upwelling is reflected in lower δ15N compared to the late Holocene. The displacement of the core of the OMZ from the region of maximum productivity in the western Arabian Sea to its present position in the northeast was established during the last 4–5 ka. This was probably caused by (i) rising oxygen consumption due to enhanced northeast monsoon driven biological productivity, in combination with (ii) reduced ventilation due to a longer residence time of OMZ waters.

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Human occupation strategies and related environmental-climate during the middle and late Holocene in central Pampas of Argentina

The Holocene ◽

10.1177/0959683618810407 ◽

2018 ◽

Vol 29 (2) ◽

pp. 244-261 ◽

Cited By ~ 4

Author(s):

Pablo G Messineo ◽

Marcela S Tonello ◽

Silvina Stutz ◽

Alfonsina Tripaldi ◽

Nahuel Scheifler ◽

...

Keyword(s):

Late Holocene ◽

Specific Activity ◽

Climatic Conditions ◽

Hard Material ◽

Human Occupation ◽

Climate Conditions ◽

Lithic Assemblage ◽

Depositional Rate ◽

Archeological Site ◽

Middle And Late Holocene

The main objective of this work is to generate and integrate interpretations of human occupation strategies and inferences of the environmental-climatic conditions in the central Pampas during the middle and late Holocene. We present a novel archeological–geological–paleoecological analysis in the area of the Cabeza de Buey lake, placed in an aeolian landscape. During the middle Holocene, two events of human occupations were recognized at Laguna Cabeza de Buey 2 archeological site. Both events present a small amount of lithic materials, a low diversity of tools and activities developed with them (principally hard material), and the hunting and primary processing of artiodactyls. These evidences suggest a locus of specific activity associated with an ephemeral human settlement under climate conditions drier than present and the presence of small, brackish, and shallow water bodies. For the late late Holocene, the hunter-gatherer occupation has a higher depositional rate of lithic assemblage, stones with diverse origins, presence of pottery fragments, a great lithic tool diversity, knapping techniques, and activities developed with these tools (processing wood, bone, hide, non-woody plant, and soft material). These evidences reveal an occupation with a higher degree of recurrence represented by a locus of multiple activities associated with a more stable landscape, such as an environment of dunes fixed by grass vegetation, and the establishment of a permanent water body. The different environmental characteristics for the middle and late Holocene in this area promoted that human groups develop two different patterns of mobility, settlement and use of space.

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Mid-to-late Holocene Temperature Evolution and Atmospheric Dynamics over Europe in Regional Model Simulations

10.5194/cp-2016-10 ◽

2016 ◽

Author(s):

Emmanuele Russo ◽

Ulrich Cubasch

Keyword(s):

Late Holocene ◽

Climate Models ◽

Climate Model ◽

Regional Climate ◽

Climatic Conditions ◽

Modes Of Variability ◽

The North ◽

Climate Reconstructions ◽

The North Atlantic Oscillation ◽

Almost All

Abstract. The improvement in resolution of climate models is always been mentioned as one of the most important factors when investigating past climatic conditions especially in order to evaluate and compare the results against proxy data. In this paper we present for the first time a set of high resolution simulations for different time slices of mid-to-late Holocene performed over Europe using a Regional Climate Model. Through a validation against a new pollen-based climate reconstructions dataset, covering almost all of Europe, we test the model performances for paleoclimatic applications and investigate the response of temperature to variations in the seasonal cycle of insolation, with the aim of clarifying earlier debated uncertainties, giving physically plausible interpretations of both the pollen data and the model results. The results reinforce previous findings showing that summertime temperatures were driven mainly by changes in insolation and that the model is too sensitive to such changes over Southern Europe, resulting in drier and warmer conditions. In winter, instead, the model does not reproduce correctly the same amplitude of changes, even if it captures the main pattern of the pollen dataset over most of the domain for the time periods under investigation. Through the analysis of variations in atmospheric circulation we suggest that, even though in some areas the discrepancies between the two datasets are most likely due to high pollen uncertanties, in general the model seems to underestimate the changes in the amplitude of the North Atlantic Oscillation, overestimating the contribution of secondary modes of variability

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Cryptic role of tetrathionate in the sulfur cycle: A study from Arabian Sea oxygen minimum zone sediments

10.1101/686469 ◽

2019 ◽

Author(s):

Subhrangshu Mandal ◽

Sabyasachi Bhattacharya ◽

Chayan Roy ◽

Moidu Jameela Rameez ◽

Jagannath Sarkar ◽

...

Keyword(s):

Arabian Sea ◽

Potential Role ◽

Sediment Cores ◽

Oxygen Minimum Zone ◽

Sulfur Cycle ◽

Metagenome Analysis ◽

Minimum Zone ◽

Oxygen Minimum

ABSTRACTTo explore the potential role of tetrathionate in the sulfur cycle of marine sediments, the population ecology of tetrathionate-forming, oxidizing, and respiring microorganisms was revealed at 15-30 cm resolution along two, ∼3-m-long, cores collected from 530- and 580-mbsl water-depths of Arabian Sea, off India’s west coast, within the oxygen minimum zone (OMZ). Metagenome analysis along the two sediment-cores revealed widespread occurrence of the structural genes that govern these metabolisms; high diversity and relative-abundance was also detected for the bacteria known to render these processes. Slurry-incubation of the sediment-samples, pure-culture isolation, and metatranscriptome analysis, corroborated thein situfunctionality of all the three metabolic-types. Geochemical analyses revealed thiosulfate (0-11.1 μM), pyrite (0.05-1.09 wt %), iron (9232-17234 ppm) and manganese (71-172 ppm) along the two sediment-cores. Pyrites (via abiotic reaction with MnO2) and thiosulfate (via oxidation by chemolithotrophic bacteria prevalentin situ) are apparently the main sources of tetrathionate in this ecosystem. Tetrathionate, in turn, can be either converted to sulfate (via oxidation by the chemolithotrophs present) or reduced back to thiosulfate (via respiration by native bacteria); 0-2.01 mM sulfide present in the sediment-cores may also reduce tetrathionate abiotically to thiosulfate and elemental sulfur. Notably tetrathionate was not detectedin situ- high microbiological and geochemical reactivity of this polythionate is apparently instrumental in the cryptic nature of its potential role as a central sulfur cycle intermediate. Biogeochemical roles of this polythionate, albeit revealed here in the context of OMZ sediments, may well extend to the sulfur cycles of other geomicrobiologically-distinct marine sediment horizons.

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Cryptic role of tetrathionate in the sulfur cycle: A study from Arabian Sea oxygen minimum zone sediments

10.5194/bg-2019-248 ◽

2019 ◽

Author(s):

Subhrangshu Mandal ◽

Sabyasachi Bhattacharya ◽

Chayan Roy ◽

Moidu Jameela Rameez ◽

Jagannath Sarkar ◽

...

Keyword(s):

Arabian Sea ◽

Potential Role ◽

Sediment Cores ◽

Oxygen Minimum Zone ◽

Sulfur Cycle ◽

Metagenome Analysis ◽

Minimum Zone ◽

Oxygen Minimum

Abstract. To explore the potential role of tetrathionate in the sulfur cycle of marine sediments, the population ecology of tetrathionate-forming, oxidizing, and respiring microorganisms was revealed at 15–30 cm resolution along two, ~ 3-m-long, cores collected from 530- and 580-mbsl water-depths of Arabian Sea, off India’s west coast, within the oxygen minimum zone (OMZ). Metagenome analysis along the two sediment-cores revealed widespread occurrence of the structural genes that govern these metabolisms; high diversity and relative-abundance was also detected for the bacteria known to render these processes. Slurry-incubation of the sediment-samples, pure-culture isolation, and metatranscriptome analysis, corroborated the in situ functionality of all the three metabolic-types. Geochemical analyses revealed thiosulfate (0–11.1 µM), pyrite (0.05–1.09 wt %), iron (9232–17234 ppm) and manganese (71–172 ppm) along the two sediment-cores. Pyrites (via abiotic reaction with MnO2) and thiosulfate (via oxidation by chemolithotrophic bacteria prevalent in situ) are apparently the main sources of tetrathionate in this ecosystem. Tetrathionate, in turn, can be either converted to sulfate (via oxidation by the chemolithotrophs present) or reduced back to thiosulfate (via respiration by native bacteria); 0–2.01 mM sulfide present in the sediment-cores may also reduce tetrathionate abiotically to thiosulfate and elemental sulfur. Notably tetrathionate was not detected in situ – high microbiological and geochemical reactivity of this polythionate is apparently instrumental in the cryptic nature of its potential role as a central sulfur cycle intermediate. Biogeochemical roles of this polythionate, albeit revealed here in the context of OMZ sediments, may well extend to the sulfur cycles of other geomicrobiologically-distinct marine sediment horizons.

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Presence of lakes and wetlands decreases resilience of jack pine ecosystems to late-Holocene climatic changes

Canadian Journal of Forest Research ◽

10.1139/cjfr-2014-0107 ◽

2014 ◽

Vol 44 (11) ◽

pp. 1331-1343 ◽

Cited By ~ 13

Author(s):

Elizabeth A. Lynch ◽

Randy Calcote ◽

Sara C. Hotchkiss ◽

Michael Tweiten

Keyword(s):

Pinus Banksiana ◽

Late Holocene ◽

Fire History ◽

Climate Changes ◽

Sediment Cores ◽

Fire Regimes ◽

Jack Pine ◽

Climatic Changes ◽

Climatic Conditions ◽

Positive Feedbacks

We reconstructed vegetation and fire histories from four sites located on a sandy outwash plain in northwestern Wisconsin (USA) to test whether lakes and wetlands have influenced how vegetation and fire regimes in pine–oak forests responded to late-Holocene climatic changes. Because of positive feedbacks between jack pine (Pinus banksiana Lamb.) and fire, communities with few fire breaks should be more resilient to changing climatic conditions. Pollen and charcoal from lake-sediment cores were used to reconstruct vegetation changes at 50- to 100-year intervals and forest fire history at decadal time scales for the past 2500 years. The presence of fire breaks affected both fire regimes and the response of vegetation to climatic changes. Areas with more fire breaks had smaller charcoal peaks and the vegetation was more responsive to climate changes. The vegetation in areas with few fire breaks was more resilient, maintaining higher amounts of jack pine and (or) red pine than the more protected sites. We interpret these findings as evidence that positive feedbacks between fire and jack pine forests stabilized vegetation at sites where fire breaks were absent, and that such sites may be relatively resilient to future climate changes, until jack pine is no longer able to regenerate under the regional climatic conditions.

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The late Pleistocene and Holocene glaciation history of sub-Antarctic South Georgia

10.5194/egusphere-egu21-6763 ◽

2021 ◽

Author(s):

Nina-Marie Lešić ◽

Katharina Streuff ◽

Gerhard Kuhn ◽

Gerhard Bohrmann ◽

Tilo von Dobeneck

Keyword(s):

Grain Size ◽

Size Distribution ◽

Late Pleistocene ◽

Late Holocene ◽

Sediment Cores ◽

Outer Shelf ◽

Sedimentation Rates ◽

Gravel Content ◽

South Georgia ◽

The Antarctic

The ice cap of the sub-Antarctic island South Georgia is influenced by the Antarctic Circumpolar Current and is hence more sensitive to changing climate than the significantly larger and more isolated Antarctic ice sheets. Furthermore, the sediment deposits in fjords and glacially eroded troughs around the island have superbly archived glacier behavior, environmental and climatic changes since the late Pleistocene. This makes South Georgia an attractive target to study past climate variability in the Southern Hemisphere. Nevertheless, the ice sheet&#8217;s extents and dynamics during the Last Glacial Maximum (LGM), the Antarctic Cold Reversal (ACR), and the Holocene deglaciation phase are still poorly understood. Although several studies on land and in marine near-shore areas of South Georgia have addressed this, only few studies are based on marine sediment cores from the continental shelf. In this study, we use ten gravity cores from three different troughs on the southern and northwestern shelf to further investigate the climatic and glaciological evolution of South Georgia during and since the LGM.Multi-proxy sedimentological analyses carried out in this study include core logging, XRF geochemical profiling, XRD analyses on bulk sediment and clay fraction, measurements of physical properties, magnetic susceptibility, grain size distribution and shear strength. For the Drygalski Trough on the southern shelf, lithofacies description reveals the deposition of stratified, predominantly sandy diamicton and greenish-grey massive to laminated, sometimes bioturbated mud with variable amounts of clasts. First radiocarbon ages from benthic foraminifera constrain the deposition of the diamicton, interpreted as waterlain till, on the outer shelf to the LGM. Inferred linear sedimentation rates attest to low sediment input on the outer shelf during the LGM (34 cm/ka) and the Holocene (23-32 cm/ka). In contrast, a higher sedimentation rate (114 cm/ka) between 14.7 and 13.7 cal ka BP is likely associated with enhanced erosion due to a possible re-advance of South Georgia&#8217;s glaciers during the ACR&#8217;s colder and wetter climate. For island-proximal cores, sedimentation rates are generally higher than on the outer shelf with rates of 80-2300 cm/ka during the Mid- to Late Holocene. This stronger fluctuation of sedimentation rates is due to higher temporal resolution of the dated sediments compared to the outer shelf. Grain-size distribution on the outer shelf shows a gravel content of 1-28 wt% in the diamicton facies from the LGM and 1-5 wt% in a sediment interval dated to 16.8 cal ka BP. This sediment interval is also characterized by a high content of pebbles, likely reflecting an increased input of IRD. The overlying ACR and Holocene show a low gravel content of 0-0.7 wt%. The diamicton suggests that ice-proximal conditions prevailed on the outer shelf during the LGM and therefore supports the theory of a shelf-wide glaciation. The combination of a low-resolution sediment core from the outer shelf and island-proximal high-resolution sediment cores has the potential to give new insights into South Georgia&#8217;s climate history from the Late Pleistocene to the Late Holocene.

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Late Quaternary glaciation and deglaciation of the Bunger Hills, Antarctica

Antarctic Science ◽

10.1017/s0954102092000658 ◽

1992 ◽

Vol 4 (4) ◽

pp. 435-446 ◽

Cited By ~ 23

Author(s):

Donald A. Adamson ◽

Eric A. Colhoun

Keyword(s):

Late Holocene ◽

Late Quaternary ◽

Ice Sheet ◽

Antarctic Ice Sheet ◽

Quaternary Glaciation ◽

Last Glaciation ◽

Coastal Inlets ◽

The Antarctic ◽

Raised Beaches ◽

Middle And Late Holocene

The Bunger Hills were covered by the Antarctic Ice Sheet during the last glaciation. During deglaciation the ice sheet margin collapsed into the marine inlets and the sea entered the oasis before 7.7 ka BP. Raised beaches occur widely below 8.5 m and indicate uplift at 1.4 m ka−1 during the middle and late Holocene. After the coastal inlets were formed, the Edisto Ice Tongue and Apfels Glacier still impinged on land margins in the west of the oasis. Two sets of marginal moraines were formed; the Older Edisto Moraines after 6.2 ka BP and the Younger Edisto Moraines during the last few centuries. The margins of the Antarctic Ice Sheet and Apfels Glacier in the south have maintained their present positions since at least 5.6 ka BP and probably 10 ka BP.

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Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal time scales

Biogeosciences Discussions ◽

10.5194/bgd-10-15455-2013 ◽

2013 ◽

Vol 10 (9) ◽

pp. 15455-15517 ◽

Cited By ~ 1

Author(s):

K. Banse ◽

S. W. A. Naqvi ◽

P. V. Narvekar ◽

J. R. Postel ◽

D. A. Jayakumar

Keyword(s):

Arabian Sea ◽

Monsoon Season ◽

Temporal Trends ◽

Southwest Monsoon ◽

Oxygen Minimum Zone ◽

Northeast Monsoon ◽

Endpoint Detection ◽

N2o Production ◽

Minimum Zone ◽

Oxygen Minimum

Abstract. The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the three oceanic OMZs, which is of global biogeochemical significance because of denitrification in the upper part leading to N2 and N2O production. The residence time of the OMZ water is believed to be less than a decade. The upper few hundred meters of this zone are nearly anoxic but non-sulfidic and still support animal (metazoan) pelagic life, possibly as a result of episodic injections of O2 by physical processes. The very low O2 values obtained with the new STOX sensor in the eastern tropical South Pacific probably also characterize the Arabian Sea OMZ, but there is no apparent reason as to why the temporal trends of the historic data should not hold. We report on discrete measurements of dissolved O2 and NO2-, besides temperature and salinity, made between 1959 and 2004 well below the tops of the sharp pycno- and oxyclines near 150, 200, 300, 400, and 500 m depth. We assemble nearly all O2 determinations (originally, 849 values, 695 in the OMZ) by the visual endpoint detection of the iodometric Winkler procedure, which in our data base yields about 0.04 mL L−1 (∼2 μM) O2 above the endpoint from modern automated titration methods. We find 632 values acceptable (480 from 150 stations in the OMZ). The data are grouped in zonally-paired boxes of 1° lat. and 2° long. centered at 8°, 10°, 12°, 15°, 18°, 20°, and 21° N along 65° E and 67° E. The latitudes of 8–12° N, outside the OMZ, are only treated in passing. The principal results are as follows: (1) an O2 climatology for the upper OMZ reveals a marked seasonality at 200 to 500 m depth with O2 levels during the northeast monsoon and spring intermonsoon season elevated over those during the southwest monsoon season (median difference, 0.08 mL L−1 [3.5 μM]). The medians of the slopes of the seasonal regressions of O2 on year for the NE and SW monsoon seasons are −0.0043 and −0.0019 mL L−1 a−1, respectively (−0.19 and −0.08 μM a−1; n = 10 and 12, differing at p = 0.01); (2) four decades of statistically significant decreases of O2 between 15° and 20° N but a trend to a similar increase near 21° N are observed. The balance of the mechanisms that more or less annually maintain the O2 levels are still uncertain. At least between 300 and 500 m the annual reconstitution of the decrease is inferred to be due to lateral, isopycnal re-supply of O2, while at 200 (250?) m it is diapycnal, most likely by eddies. Similarly, recent models show large vertical advection of O2 well below the pycno-cum-oxycline. The spatial (within drift stations) and temporal (daily) variability in hydrography and chemistry is large also below the principal pycnocline. The seasonal change of hydrography is considerable even at 500 m. There is no trend in the redox environment for a quarter of a century at a GEOSECS station near 20° N. In the entire OMZ the slopes on year within seasons for the quite variable NO2- (taken as an indicator of active denitrification) do not show a clear pattern. Also, future O2 or nutrient budgets for the OMZ should not be based on single cruises or sections obtained during one season only. Steady state cannot be assumed any longer for the intermediate layers of the central Arabian Sea.

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Antarctic ice-sheet response to atmospheric CO2 and insolation in the Middle Miocene

Climate of the Past ◽

10.5194/cp-5-633-2009 ◽

2009 ◽

Vol 5 (4) ◽

pp. 633-646 ◽

Cited By ~ 29

Author(s):

P. M. Langebroek ◽

A. Paul ◽

M. Schulz

Keyword(s):

Middle Miocene ◽

Climate Model ◽

Sediment Cores ◽

Ice Sheets ◽

Ice Sheet ◽

Rapid Expansion ◽

Atmospheric Co2 ◽

Antarctic Ice Sheet ◽

Deep Sea Sediment ◽

The Antarctic

Abstract. Foraminiferal oxygen isotopes from deep-sea sediment cores suggest that a rapid expansion of the Antarctic ice sheet took place in the Middle Miocene around 13.9 million years ago. The origin for this transition is still not understood satisfactorily. One possible cause is a drop in the partial pressure of atmospheric carbon dioxide (pCO2) in combination with orbital forcing. A complication is the large uncertainty in the magnitude and timing of the reconstructed pCO2 variability and additionally the low temporal resolution of the available pCO2 records in the Middle Miocene. We used an ice sheet-climate model of reduced complexity to assess variations in Antarctic ice sheet volume induced by pCO2 and insolation forcing in the Middle Miocene. The ice-sheet sensitivity to atmospheric CO2 was tested for several scenarios with constant pCO2 forcing or a regular decrease in pCO2. This showed that small, ephemeral ice sheets existed under relatively high atmospheric CO2 conditions (between 640–900 ppm), whereas more stable, large ice sheets occurred when pCO2 was less than ~600 ppm. The main result of this study is that the pCO2-level must have declined just before or during the period of oxygen-isotope increase, thereby crossing a pCO2 glaciation threshold of around 615 ppm. After the decline, the exact timing of the Antarctic ice-sheet expansion depends also on the relative minimum in summer insolation at approximately 13.89 million years ago. Although the mechanisms described appear to be robust, the exact values of the pCO2 thresholds are likely to be model-dependent.

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Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification