A 370ka Foraminifera Based Dissolution Record of IODP Site U1446 in Northwestern Bay of Bengal

Indian Summer Monsoon may be the most important coupled, bi-hemispheric system in tropical area influencing both the northern Indian Ocean and the Asian continent. To fill in gaps in monsoon core region (Bay of Bengal) reconstruction, site U1446 (19°5´N, 85°44´E, 1430m water depth) was retrieved from northwestern Bay of Bengal, during IODP expedition 353. We observed strong dissolution phenomena from foraminiferal records of U1446. To reconstruct the dissolution variation and understand its mechanism in Bay of Bengal, we studied a ~370ka foraminiferal record of U1446. Multiply dissolution proxies (Fragmentation Index, Globorotalia menardii Fragmentation Index, Hoeglundina elegans Preservation Index, Foraminifera Dissolution Index, percentage of Pulleniatina obliquiloculata, and ratio of benthic/ whole foraminifer) and principal components analysis (PCA) were used to reconstruct a calcite dissolution variation of U1446.

Chronostratigraphy of U1446 was established based on Toba ash, the top of Globigerinoides ruber rosa, and the correlation of benthic foraminifer Uvigerina spp. δ¹⁸O values (Clemens, unpublished data) with LR04. All dissolution proxies indicate relatively strong calcite dissolution during interglacial periods and weak during glacial periods. Two principle components (PC1, PC2) were extracted from the six individual dissolution proxies using PCA. Based on their correlation with foraminiferal fluxes, we assumed that PC1 represent the dissolution variation of U1446. Of all proxies, non-assemblage based proxies (Fragmentation Index, G. menardii Fragmentation Index, H. elegans Preservation Index, and benthic/ whole foraminifer ratio) show high loadings in PC1 and low in PC2 whereas assemblage based proxies (Foraminifera Dissolution Index, P. obliquiloculata %) show high loadings in PC2 and low in PC1. This suggest that calcite dissolution in U1446 didn't have the prospective selective dissolution effect on foraminiferal assemblage. Spectral analysis of PC1 show strong ~23ka (>99%), ~100ka (>99%) cyclicity and relatively weak ~41ka (>90%) cyclicity. This indicate that calcite dissolution in U1446 was mainly driven by Earth's orbital parameter on orbital scale, eccentricity and precession play a more important role than obliquity in controlling dissolution variation.