Core-top Evaluation of Planktonic Foraminiferal Mg/Ca and Leaf wax n-alkanoic Acids as Proxies for the Indian Monsoon

Reconstructions of the strength of the Indian Monsoon using marine sediments have historically relied on the percentage of the planktonic foraminifer Globigerina bulloides, which ideally records the strength of monsoon--induced upwelling in the Arabian Sea. While extremely useful, it is unclear whether subtle variations in G. bulloides abundance can be quantitatively linked to subtle changes in the monsoon. For example, G. bulloides abundances saturate above ∼35--40%, which suggests potential problems with quantitative interpretations of this proxy. In this study, we use 22 core--top sediment samples geographically distributed throughout the Arabian Sea to assess the paleoceanographic potential of three alternative proxies for monsoon strength: planktonic foraminiferal Mg/Ca, leaf wax n--alkanoic acid concentrations and leaf wax δ ¹³C. Strong upwelling along the Oman Margin during the Indian summer monsoon cools sea surface temperatures (SSTs) along the coast of Oman by several degress relative to the non--upwelling season and to non--upwelling areas. Because the Mg/Ca ratio of foraminiferal shells is temperature dependent, this proxy may provide a more rigorous estimate of the strength of upwelling than the abundance of G. bulloides. We also assess the potential use of long--chain (C₂₄--C₃₂) leaf wax n--alkanoic acids, derived from terrestrial plants, as indicators of monsoon wind strength and direction. Winter and summer Indian monsoon winds have the potential to deliver leaf wax n--alkanoic acids to the Arabian Sea from Asia and Africa, respectively. During the summer monsoon, leaf waxes should be largely derived from Ethiopia, where desert grasslands and C4 plants dominate the vegetation. During the winter monsoon, leaf waxes should be derived from Asia, where C3 plants are dominant. Thus, both the concentration of leaf waxes and their carbon isotopic composition should yield information regarding the sources of these compounds to sediments. In addition to presenting modern (i.e. core--top) data, we will discuss the utility of planktonic foraminiferal Mg/Ca and leaf wax n--alkanoic acids as potential indicators of monsoon strength in the past.