High-Resolution Monsoon Records Since Last Glacial Maximum: A Comparison of Marine and Terrestrial Paleoarchives from South Asia

Table 2

A few of the important proxies/archives used for monsoon reconstruction and their significance.

S. No.	Proxies/archives discussed in the present work	Significances of each proxies
(1)	Oxygen isotopes of Speleothems and Foraminifera	Oxygen isotopes of foraminifera reflect the isotopic composition of the seawater that depends on salinity and temperature. Eastern Arabian Sea receives abundant fresh water as either direct precipitation or runoff from the adjacent Western Ghats during the Southwest monsoon. This reduces the sea surface salinity (SSS) that is reflected in negative excursion in the oxygen isotopic composition of planktic foraminifera.
(2)	Globigerina bulloides abundance in tropical oceans	The spatial distribution of Globigerina bulloides in the world ocean shows that it is dominant in temperate subpolar water mass and thus the only likely cause for high abundance in low latitude areas (tropical oceans) has been upwelling induced productivity. The initiation of upwelling in the western Arabian Sea and the subsequent increase in Globigerina bulloides flux indicates that foraminiferal population respond within a few weeks to changes in near surface hydrography, which has been demonstrated in studies from Western Arabian Sea Sediment Trap data. The enhanced upwelling in the Arabian Sea, especially western region, is strongly correlated to Southwest monsoon.
(3)	Carbon isotopes of foraminifera	Kinetic isotope effects during photosynthesis cause preferential uptake of ^12C in the organic matter, which enriches the ambient dissolved bicarbonate in heavier isotopes (^13C). The foraminifera secreting calcareous shells in equilibrium with the ambient water will record these isotopic signatures. Thus a higher δ ^13C value probably corresponds to an enhanced rate of photosynthesis in the euphotic layer that indicates an increase in productivity relatable to stronger monsoon.
(4)	Nitrogen Isotopes of sedimentary Organic matter	Due to lack of oxygen in Oxygen Minima Zone, the anaerobic bacteria utilize NO3 ^− for the decomposition of organic matter. During this process they preferentially consume NO3 ^− with lighter isotope (^14N), thus enriching the residual nitrate in the heavier isotope, which gets upwelled to the sea surface and is taken by the organisms as a nutrient. This enriched nitrogen isotopic signature is preserved even when the organic matter settles down and gets preserved in sea sediments. Thus a high δ ^15N can be related to increased denitrification, which in turn is controlled by the productivity increase relatable to monsoon strength.
(5)	Total Organic Carbon & Inorganic Carbon	Total Organic Carbon (TOC) preserved in the sea sediments is derived from the particulate organic carbon (POC, the carbon content of particulate organic matter) and is a manifestation of the overhead primary productivity if there are no alterations after the deposition. The overhead rain of calcitic shells is a major constituent of the sea sediments. It has been observed that during the monsoon season in the Arabian Sea, 50–60% of the total flux to the bottom is composed of calcitic material. Thus calcium carbonate percentage in the sea sediments can indicate the overhead productivity provided the core has been raised from depths above the lysocline (~3800 m in the Arabian Sea) and there is no contamination from the terrigenous inputs