Decadal-resolution record of winter monsoon intensity over the last two millennia from planktic foraminiferal assemblages in the northeastern Arabian Sea

Abstract

The Indian monsoon system is an important climate feature of the northern Indian Ocean. Small variations of the wind and precipitation patterns have fundamental influence on the societal, agricultural, and economic development of India and its neighboring countries. To understand current trends, sensitivity to forcing, or natural variation, records beyond the instrumental period are needed. However, high-resolution archives of past winter monsoon variability are scarce. One potential archive of such records are marine sediments deposited on the continental slope in the NE Arabian Sea, an area where present-day conditions are dominated by the winter monsoon. In this region, winter monsoon conditions lead to distinctive changes in surface water properties, affecting marine plankton communities that are deposited in the sediment. Using planktic foraminifera as a sensitive and well-preserved plankton group, we first characterize the response of their species distribution on environmental gradients from a dataset of surface sediment samples in the tropical and sub-tropical Indian Ocean. Transfer functions for quantitative paleoenvironmental reconstructions were applied to a decadal-scale record of assemblage counts from the Pakistan Margin spanning the last 2000 years. The reconstructed temperature record reveals an intensification of winter monsoon intensity near the year 100 CE. Prior to this transition, winter temperatures were >1.5°C warmer than today. Conditions similar to the present seem to have established after 450 CE, interrupted by a singular event near 950 CE with warmer temperatures and accordingly weak winter monsoon. Frequency analysis revealed significant 75-, 40-, and 37-year cycles, which are known from decadal-to centennial-scale resolution records of Indian summer monsoon variability and interpreted as solar irradiance forcing. Our first independent record of Indian winter monsoon activity confirms that winter and summer monsoons were modulated on the same frequency bands and thus indicates that both monsoon systems are likely controlled by the same driving force.

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

“…TEX 86 -and alkenone-based SST reconstructions of cores NIOP905 and SO42-KL74 were shown to differ in magnitude and phase as TEX 86 temperature reconstructions seem to have a SW monsoon bias (Huguet et al, 2006). Munz et al (2015) showed that winter temper-atures derived from planktic foraminiferal assemblages had stronger amplitudes than the alkenone-based annual average SST reconstructions in SO130-275KL of Böll et al (2014). Two of the records used in this study have both, alkenone and Mg / Ca temperature.…”

Decadal resolution record of Oman upwelling indicates solar forcing of the Indian summer monsoon (9–6 ka)

“…The alongshore surface currents induce an offshore-directed deflection of surface waters and an upwelling of deeper, cold and nutrient-rich subsurface waters through the process of Ekman pumping. During boreal winter (December-March), weaker, dry and cold northeasterly winds (IWM, Indian winter monsoon) lead to increased surface water cooling and evaporation, especially in the northeastern part of the basin, where the IWM enables deep convective mixing and a second productivity peak during winter (Madhupratap et al, 1996;Schulz et al, 2002;Munz et al, 2015. Enhanced nutrient availability during summer upwelling nourishes primary productivity within the photic zone near the Somali and Omani coasts.…”

“…• N and 33 to 119 • E recently compiled by Munz et al (2015). To enhance the signal-to-noise ratio, rare species with < 0.5 % average abundance were removed from the calibration dataset prior to further analyses (Kucera et al, 2005), which resulted in the exclusion of 16 rare species out of 33 taxa.…”

“…This removes noisy effects introduced by modern samples covering ecological conditions outside the range of the fossil samples (Kucera et al, 2005). We followed the approach presented in Munz et al (2015) and restricted the calibration dataset to samples that had the same range along the first and second axes of a joint principal component analysis together with the fossil dataset (using squareroot-transformed species abundances to dampen the effect of a few dominant taxa). The resulting subsampled dataset used for further analyses contained n = 96 modern surface samples (Fig.…”

“…Reconstructions are based on a comparison of Mg / Ca measurements of the upwellingrelated species Globigerina bulloides and on transfer functions based on planktic foraminiferal (PF) census counts. To this end we first evaluate species response to potential ecological driving parameters and convert fossil assemblage counts to summer SST using a regionally validated transfer function (Munz et al, 2015). We test the ecological importance and statistical significance of the reconstructions and evaluate summer SST variability in comparison to other proxy records.…”