Impact of riverine sediment mineralogy on seawater Nd isotope compositions in the northeastern part of the Indian Ocean during the last two glacial cycles

Abstract

Radiogenic neodymium isotope compositions (εNd) are used as a tracer for water mass circulation and continental weathering at different timescales. However, uncertainties remain in the relative roles of these two factors in driving past seawater εNd variability in settings under the influence of terrestrial or riverine sediment inputs. In this study, Nd isotopes of mixed planktonic foraminifera species and δ¹⁸O and δ¹³C of Cibicidoides wuellerstorfi were analyzed on three cores from the northeastern Indian Ocean to better assess the impact of lithogenic inputs from Himalayan rivers and deep-water hydrological changes on the past εNd distribution in the Bay of Bengal (BoB). Our εNd data indicate relatively homogenous and radiogenic values (from -8.4 to -7.5) during glacial periods in the BoB, similar to the composition of glacial water masses of the Southern Ocean. In contrast, interglacials were characterized by more unradiogenic εNd and a pronounced north-south gradient of ∼4.5 εNd units (from -12.9 to -8.5) in bottom water, similar to the present-day distribution in the BoB, pointing to a strong lithogenic control by seawater-particulate interactions. Notably, this significant decoupling of the local Nd isotope signature from the Southern Ocean composition occurred when Himalayan riverine inputs were dominated by the erosion of Indo-Gangetic plain soils during interglacial periods, whereas the preferential delivery of fresh primary mineral assemblages during glacial periods appears to have had little impact on Nd exchange with seawater. These findings provide direct evidence that the degree of seawater-particulate exchange at continental margins is governed by the mineralogy of riverine inputs, with further implications for the use of Nd isotopes as palaeoceanographic tracers.