scholarly journals A multi-proxy analysis of late Quaternary Indian monsoon dynamics for the Maldives, Inner Sea

2017 ◽  
Author(s):  
Dorothea Bunzel ◽  
Gerhard Schmiedl ◽  
Sebastian Lindhorst ◽  
Andreas Mackensen ◽  
Jesús Reolid ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Organic Carbon ◽  
Arabian Sea ◽  
Bottom Water ◽  
Indian Monsoon ◽  
Late Quaternary ◽  
Intermediate Water ◽  
Combined Effects ◽  
Proxy Data ◽  
Data Record

Abstract. We present a detailed multi-proxy data record to reveal the late Quaternary changes in marine sedimentation and biogeochemical processes of the upper bathyal Maldives (equatorial Indian Ocean) and how they are related to the benthic ecosystem dynamics. We investigated the sediment core SO-236-052-4 from the central part of the Inner Sea, Maldives, focusing on Fe / Ca and Si / Ca ratios as proxies for terrigenous sediment delivery, as well as Total Organic Carbon (TOC) and Ba / Ca ratios as proxies for marine productivity. Benthic foraminiferal fauna distributions, sortable silt records and stable oxygen and carbon isotope analyses were used for reconstructing the past ecosystem, as well as changes in the intermediate water circulation, bottom water current velocity and oxygenation. This multi-proxy data record shows an enhanced dust supply during the glacial intervals, represented by increased Fe / Ca and Si / Ca ratios, an overall coarsening of the sediment and increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilisation during the cold phases resulted in increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas and the distribution of certain detritus and suspension feeders were fostered. The difference in the stable carbon isotope signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 μmol kg−1. The pattern of oxygen changes resembles that from the deep Arabian Sea suggesting an expansion of the Oxygen Minimum Zone (OMZ) from the Arabian Sea into the tropical Indian Ocean, further controlled by the inflow of the Antarctic Intermediate Water (AAIW). The precessional circulation pattern of the bottom water oxygenation is overprinted by glacial-/interglacial changes resulting in a long phase of reduced ventilation during the last glacial period. The latter process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and restriction of bathyal environments of the Inner Sea of the Maldives during sea-level lowstands. Thus, this multi-proxy record provide a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital time-scale.

scholarly journals A multi-proxy analysis of Late Quaternary ocean and climate variability for the Maldives, Inner Sea

2017 ◽  
Vol 13 (12) ◽  
pp. 1791-1813 ◽  
Author(s):  
Dorothea Bunzel ◽  
Gerhard Schmiedl ◽  
Sebastian Lindhorst ◽  
Andreas Mackensen ◽  
Jesús Reolid ◽  
...  
Keyword(s):  
Oxygen Consumption ◽  
Indian Ocean ◽  
Organic Carbon ◽  
Sea Level ◽  
Benthic Foraminifera ◽  
Arabian Sea ◽  
Bottom Water ◽  
Winter Monsoon ◽  
Intermediate Water ◽  
Combined Effects

Abstract. As a natural sediment trap, the marine sediments of the sheltered central part of the Maldives Inner Sea represent an exceptional archive for paleoenvironmental and climate changes in the equatorial Indian Ocean. To evaluate the complex interplay between high-latitude and monsoonal climate variability, related dust fluxes, and regional oceanographic responses, we focused on Fe ∕ Al, Ti ∕ Al and Si ∕ Ca ratios as proxies for terrigenous sediment delivery and total organic carbon (TOC) and Br XRF counts as proxies for marine productivity. Benthic foraminiferal fauna distributions, grain size and stable δ18O and δ13C data were used for evaluating changes in the benthic ecosystem and changes in the intermediate water circulation, bottom water current velocity and oxygenation. Our multi-proxy data record reveals an enhanced dust supply during the glacial intervals, causing elevated Fe ∕ Al and Si ∕ Ca ratios, an overall coarsening of the sediment and an increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilization during the cold phases resulted in an increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas with certain detritus and suspension feeders was fostered. The difference in the δ13C signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 µmol kg−1 during this time. The precessional fluctuation pattern of oxygen changes resembles that from the deep Arabian Sea, suggesting an expansion of the oxygen minimum zone (OMZ) from the Arabian Sea into the tropical Indian Ocean with a probable regional signal of strengthened winter-monsoon-induced organic matter fluxes and oxygen consumption further controlled by the varying inflow intensity of the Antarctic Intermediate Water (AAIW). In addition, the bottom water oxygenation pattern of the Maldives Inner Sea reveals a long phase of reduced ventilation during the last glacial period. This process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and the restriction of upper bathyal environments in the Inner Sea during sea-level lowstands. Thus, our multi-proxy record reflects a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital timescale.

scholarly journals Changes in productivity and intermediate circulation in the northern Indian Ocean since the last deglaciation: new insights from benthic foraminiferal Cd /thinsp;Ca records and benthic assemblage analyses

2020 ◽  
Author(s):  
Ruifang Ma ◽  
Sophie Sépulcre ◽  
Laetitia Licari ◽  
Frédéric Haurine ◽  
Franck Bassinot ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Primary Productivity ◽  
Arabian Sea ◽  
Bottom Water ◽  
Geochemical Data ◽  
Intermediate Water ◽  
Last Deglaciation ◽  
Northern Indian Ocean ◽  
Surface Productivity ◽  
The Last Deglaciation

Abstract. We have measured Cd / Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cdw records estimated from the benthic Cd / Ca reflect past changes in surface productivity and/or intermediate-bottom water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, the Heinrich Stadial 1 and Younger Dryas (HS1 and YD, respectively) millennial-scale events were marked by a decrease in Cdw values, indicating an enhanced ventilation of intermediate-bottom water masses. Benthic foraminifer assemblages indicate that surface primary productivity was low during the early Holocene (from 10 to 6 cal kyr BP), resulting in low intermediate water Cdw at both sites. From ~ 5.2 to 2.4 cal kyr BP, the benthic foraminiferal assemblages indicate meso- to eutrophic intermediate water conditions, which correspond to high surface productivity. This is consistent with a significant increase in the intermediate water Cdw in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediate water Cdw records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate water Cdw by monsoon-induced changes in upper water stratification and surface primary productivity.

Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations

2020 ◽  
Author(s):  
Takeshi Izumo ◽  
Maratt Satheesan Swathi ◽  
Matthieu Lengaigne ◽  
Jérôme Vialard ◽  
Dr Ramesh Kumar
Keyword(s):  
Indian Ocean ◽  
Indian Summer Monsoon ◽  
Arabian Sea ◽  
Large Scale ◽  
Indian Monsoon ◽  
Westerly Wind ◽  
Low Level ◽  
Intraseasonal Variations ◽  
The Indian Ocean ◽  
Low Level Jet

<p>A strong Low-Level Jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations  only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmosphericdeep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific / North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.</p><p> </p><p>Related reference: Swathi M.S, Takeshi Izumo, Matthieu Lengaigne, Jérôme Vialard and M.R. Ramesh Kumar:Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, accepted in Climate Dynamics.</p>

Late-Quaternary biogenic productivity and organic carbon deposition in the eastern Arabian Sea

2003 ◽  
Vol 197 (1-2) ◽  
pp. 43-60 ◽  
Author(s):  
Rajesh Agnihotri ◽  
M.M Sarin ◽  
B.L.K Somayajulu ◽  
A.J.T Jull ◽  
G.S Burr
Keyword(s):  
Organic Carbon ◽  
Arabian Sea ◽  
Carbon Deposition ◽  
Late Quaternary ◽  
Eastern Arabian Sea

scholarly journals The Role of the Western Arabian Sea Upwelling in Indian Monsoon Rainfall Variability

2008 ◽  
Vol 21 (21) ◽  
pp. 5603-5623 ◽  
Author(s):  
Takeshi Izumo ◽  
Clémentde Boyer Montégut ◽  
Jing-Jia Luo ◽  
Swadhin K. Behera ◽  
Sébastien Masson ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
Arabian Sea ◽  
Monsoon Rainfall ◽  
Indian Monsoon ◽  
Indian Summer Monsoon Rainfall ◽  
Summer Monsoon Rainfall ◽  
Indian Monsoon Rainfall ◽  
Precipitation Anomalies

Abstract The Indian summer monsoon rainfall has complex, regionally heterogeneous, interannual variations with huge socioeconomic impacts, but the underlying mechanisms remain uncertain. The upwelling along the Somalia and Oman coasts starts in late spring, peaks during the summer monsoon, and strongly cools the sea surface temperature (SST) in the western Arabian Sea. They restrict the westward extent of the Indian Ocean warm pool, which is the main moisture source for the monsoon rainfall. Thus, variations of the Somalia–Oman upwelling can have significant impacts on the moisture transport toward India. Here the authors use both observations and an advanced coupled atmosphere–ocean general circulation model to show that a decrease in upwelling strengthens monsoon rainfall along the west coast of India by increasing the SST along the Somalia–Oman coasts, and thus local evaporation and water vapor transport toward the Indian Western Ghats (mountains). Further observational analysis reveals that such decreases in upwelling are caused by anomalously weak southwesterly winds in late spring over the Arabian Sea that are due to warm SST/increased precipitation anomalies over the Seychelles–Chagos thermocline ridge of the southwestern Indian Ocean (and vice versa for years with strong upwelling/weak west Indian summer monsoon rainfall). The latter SST/precipitation anomalies are often related to El Niño conditions and the strength of the Indonesian–Australian monsoon during the previous winter. This sheds new light on the ability to forecast the poorly predicted Indian monsoon rainfall on a regional scale, helped by a proper ocean observing/forecasting system in the western tropical Indian Ocean.

scholarly journals Late Quaternary insolation forcing on total organic carbon and C37alkenone variations in the Arabian Sea

2000 ◽  
Vol 15 (3) ◽  
pp. 307-321 ◽  
Author(s):  
Dörte Budziak ◽  
Ralph R. Schneider ◽  
Frauke Rostek ◽  
Peter J. Müller ◽  
Edouard Bard ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Total Organic Carbon ◽  
Arabian Sea ◽  
Late Quaternary ◽  
Insolation Forcing

Corrigendum to ‘Late Quaternary biogenic productivity and organic carbon deposition in the eastern Arabian Sea’

2004 ◽  
Vol 202 (3-4) ◽  
pp. 353
Author(s):  
Rajesh Agnihotri ◽  
M.M. Sarin ◽  
B.L.K. Somayajulu ◽  
A.J.T. Jull ◽  
G.S. Burr ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Arabian Sea ◽  
Carbon Deposition ◽  
Late Quaternary ◽  
Eastern Arabian Sea

scholarly journals Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean

2019 ◽  
Vol 16 (13) ◽  
pp. 2715-2732 ◽  
Author(s):  
Natalie C. Harms ◽  
Niko Lahajnar ◽  
Birgit Gaye ◽  
Tim Rixen ◽  
Kirstin Dähnke ◽  
...  
Keyword(s):  
Organic Matter ◽  
Indian Ocean ◽  
Southern Ocean ◽  
Surface Waters ◽  
N2 Fixation ◽  
Arabian Sea ◽  
Isotope Composition ◽  
Water Masses ◽  
Southern Indian Ocean ◽  
Intermediate Water

Abstract. The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of < 3 and < 0.3 µmol kg−1, respectively (< 300 m; σ < 26.4 kg−1 m−3). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ15N > 7 ‰; δ18O > 4 ‰) at 400–500 m (26.6–26.7 kg−1 m−3), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg−1 m−3) of the gyre, indicated by an N deficit (N* ∼-1 to −4 µmol kg−1) and by elevated isotopic ratios of nitrate (δ15N > 7 ‰; δ18O > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of < 3 ‰ within the SAMW. In contrast, remineralization of 15N-enriched organic matter from the Arabian Sea indicates higher Δ(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of > 2 µmol kg−1 and a relatively low Δ(15–18) value of < 3 ‰. A mass and isotope budget implies that at least 32 %–34 % of the nitrate in the upper ocean between 20.36 and 23.91∘ S is provided from newly fixed nitrogen, whereas N2 fixation appears to be limited by iron or temperature south of 26∘ S.

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