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

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 δ 18 O and δ 13 C 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 δ 13 C signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between ap-proximately 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.

Glacial CO<sub>2</sub> decrease and deep-water deoxygenation by iron fertilization from glaciogenic dust

“…LGM_glac3% (red), and LGM_all (blue). Triangles show proxy records of quantitative O2 change from (Hoogakker et al, 2015(Hoogakker et al, , 2018Gottschalk et al, 2016;Lu et al, 2016;Bunzel et al, 2017;Umling and Thunell, 2018;Schmiedl and Mackensen, 2006) (Bopp et al, 2013), observed values from WOA2009 (Garcia et al, 2010a) are used for the present. For the LGM simulations, we combine the observed values with the modelled changes.…”

“…We performed several sensitivity experiments listed in Table 1 to explore the contribution of changes in atmospheric dust and nutrient inventory on glacial carbon and oxygen cycles. Moreover, our modeled oxygen changes are compared to recently reported qualitative (Jaccard and Galbraith, 2012) and quantitative reconstructions (Hoogakker et al, 2015(Hoogakker et al, , 2018Gottschalk et al, 2016;Lu et al, 2016;Bunzel et al, 2017;Umling and Thunell, 2018;Schmiedl and Mackensen, 2006) to evaluate the simulated accumulation of respired carbon. Our simulation show that enhanced efficiency of biological pump associated with glaciogenic dust and increased nutrient inventory plays a crucial role in the glacial CO2 decrease and deep-water deoxygenation.…”

“…Several recent studies using δ 13 C in benthic foraminiferal or iodine-to-calcium ratios in planktonic foraminifera, were able to quantify changes in oxygen concentration (Hoogakker et al, 2015(Hoogakker et al, , 2018Umling and Thunell, 2018;Gottschalk et al, 2016;Lu et al, 2016;Bunzel et al, 2017;Schmiedl and Mackensen, 2006). These quantitative proxy data provide firmer constraints on accumulation of respired carbon, such that quantitative model-proxy comparison of oxygen change is very useful for quantifying the contribution of the biological pump to glacial CO2 drawdown.…”

Facies and sedimentology of a carbonate delta drift (Miocene, Maldives)

“…The modern platform configuration was established during the late Middle Miocene at ca 13 Ma with atoll growth paralleled by partial bank drowning and drift deposition (Purdy & Bertram, ; Aubert & Droxler, ; Betzler et al ., , , ; Lüdmann et al ., ). This transition from a sea‐level to current‐controlled system was attributed to the onset and/or intensification of the Indian monsoon (Betzler et al ., , , ,b, ; Lüdmann et al ., ; Bunzel et al ., ; Reolid et al ., ). Since the Middle Miocene the Inner Sea was gradually infilled by ten sequences comprised of delta drifts, elongated mounded drifts and clinoform drift bodies (Lüdmann et al ., , ).…”

The ichnology of carbonate drifts

“…Starting with seismic sequence DS6, at the end of the Messinian, the opening of a southern gateway introduced northward flow of bottom currents into the Inner Sea, resulting in the deposition of giant elongated drifts attached to the eastern flank of the basin, filling it from east to west (Lüdmann et al ., ). Detached drifts form at the downcurrent flanks of the atolls (Betzler et al ., , ) and the currents controlling the drift deposition in the Maldives prevail during the Quaternary (Betzler et al ., ; Bunzel et al ., ; Reolid et al ., ). The water depth at which the drifts were deposited changed through time from ca 50 m at the time of the delta drift deposition (Reolid et al ., ) to ca 500 m in present day (Betzler et al ., ).…”