Assessment and calibration of TEX86 paleothermometry in the Sea of Okhotsk and sub-polar North Pacific region: Implications for paleoceanography

Abstract

We measured TEX86 indices in surface sediments and sediment cores from marginal seas and open ocean sites in the North Pacific to test the accuracy of TEX86 paleothermometry in the region. Application of the global core top TEX86L and TEX86H calibrations in the Sea of Okhotsk and northern North Pacific sub-polar region results in over- and under-estimations of TEX86 derived SSTs in surface sediments in deep-ocean and coastal areas, respectively. Further challenges apparent in applying TEX86L and TEX86H in late Quaternary sediment cores from the Okhotsk Sea (MR06-04 PC7) and northwestern Pacific (MR00-K03 PC1) are unrealistically warm Holocene mean annual reconstructed temperatures (TEX86L) and excessively cold Glacial temperatures (TEX86H), compared with other high-latitude records and climate models. Application of a calibration based on a sub-set of deep (<1000m) core-top sites appears to improve temperature reconstructions in the Holocene, but still yields underestimation of SSTs (mean annual=−5°C for TEX86H) in the Glacial period in the Okhotsk Sea core MR06-04 PC7. However, a regional comparison of Okhotsk Sea and sub-polar surface sediment TEX86L values with observational sea-water temperatures at different seasons and water depths reveals that an optimized positive correlation is obtained with summer sub-surface temperatures (August, 20m depth). The unique hydrographic setting of the Okhotsk Sea region likely exerts a strong control on the ecology of GDGT producers (assumed to be primarily be marine Thaumarchaeota), resulting in highly season and depth specific export production and a distinct regional gradient in surface sediment TEX86 values. When our new regional TEX86L calibration is applied to the sediment cores, TEX86L and alkenone U37K′ temperature estimates overlap in the Holocene and TEX86L produces plausible LGM to Holocene temperature reconstructions. Our study provides a region specific calibration and confirms the importance of acquiring local calibration equations, to optimize TEX86 paleothermometry for specific regions and ocean basins.