Varying responses to Indian monsoons during the past 220 kyr recorded in deep‐sea sediments in inner and outer regions of the Gulf of Aden

Abstract

Although the climate in the Arabian Sea is controlled primarily by the southwest monsoon, the southwest monsoon has little influence in the Gulf of Aden. To examine the different responses to monsoons between the Gulf of Aden and areas outside the gulf, a comprehensive data set of bulk organic and inorganic geochemistry, sea surface temperature, and δ15N of pheopigments was obtained from deep‐sea sediment cores recovered from inner and outer regions of the gulf. The results indicated that during the past 220 kyr, the influence of the southwest monsoon was stronger in the outer region of the gulf than in the inner region, which implies that the southwest monsoon trajectory has not changed substantially during that time period. Furthermore, influxes of O2‐depleted water from the Southern Ocean and the lateral advection of upwelled seawater also had limited influence in the inner region. In contrast, concentrations of lithogenic materials transported by the southwest monsoon were similar in the two regions. δ15N of pheopigments indicated that during the last glacial maximum, the southwest monsoon was weaker and the northeast monsoon was stronger than at present. A stronger southwest monsoon during interglacials enhanced primary productivity and may have caused anoxic conditions to develop in the Arabian Sea, as indicated by redox proxies in the outer region. Anoxic conditions also formed during MIS 3, but no increase in the primary productivity is indicated; therefore, another mechanism, such as an influx of O2‐depleted water from the Southern Ocean, may have been the cause.

Variations of the Somali upwelling since 18.5 ka BP and its relationship with southwest monsoon rainfall

“…6a), which is attributed to the effect of the northern limit of the SWM, and is observed at the study site by way of a subsequent increase in the Somali upwelling strength. This is further supported by the Zr / Hf ratios in two independent sediment cores collected near the study site, which show an increasing flux of windborne dust from the Horn of Africa (an indicator of the SWM) at the onset of the B/A (Sirocko et al, 2000;Isaji et al, 2015). The reduction in the TEX 86 SST during the B/A in the Somali Basin (Huguet et al, 2006) also suggests increased upwelling (Fig.…”

Efficient recycling of nutrients in modern and past hypersaline environments

“…Significantly, the isotopic signatures in modern hypersaline environments can be used for interpreting the δ 15 N signatures of the analogous ancient environments. In particular, the isotopic compositions of chloropigments (chlorophylls and bacteriochlorophylls) record primary signals of the physiology of phototrophs and the nitrogen substrates that they assimilate 16–18 . Moreover, the preservation of chloropigments as geoporphyrins in sediments on a geological timescale make them an ideal tool for reconstructing past nitrogen cycles 19–22 .…”

Sea Level Change Controlled the Sedimentary Processes at the Makran Continental Margin Over the Past 13,000 yr

“…Surface circulation also shows seasonal changes under the influence of a strong monsoon in the Arabian Sea (Shetye et al, 1994; Yu et al, 2019). In addition to the seasonally reversing monsoon winds, a northwesterly wind driven by a pressure gradient between the Iranian highlands and the Arabian lowlands blows across the Arabian Peninsula and is strongest between the summer and winter monsoon seasons (Isaji et al, 2015).…”