Is strontium isotope record a strict proxy for chemical weathering rates during the late Miocene (Ocean Drilling Program Site 758A)?

Abstract

The strontium isotope ratios are examined in the shells of planktic foraminifer Orbulina universa in 20 samples from the late Miocene (8.03-6.36 Ma) sequence of Ocean Drilling Program (ODP) Site 758A, northeastern Indian Ocean. The Sr isotope ratios increased between 7.91 and 7.36 Ma and in the latest Miocene (6.48-6.36 Ma) reflecting high riverine flux from increased chemical weathering and high precipitation in the Himalayan region. The ⁸⁷Sr/⁸⁶Sr values decreased during 7.36 to 6.48 Ma coincident with the main phase of Chron-6 negative carbon shift. The carbon shift reflects high delivery of nutrients to the oceans. This contradicts the use of Sr isotope record as a strict proxy for global chemical weathering rates. The Sr isotope ratio should be used with great care in understanding the climatic- tectonic connections.