Orbitally-resolved Pliocene environmental variability at North Pacific ODP 883

Abstract:

The North Pacific underwent dramatic oceanographic changes during the Mid Pliocene to Early Pleistocene interval. Primary productivity crashed due to water column stratification and formation of the modern halocline. This was accompanied by crashes in carbonate and opal burial, the former of which is thought to be due to shutdown of Pacific Meridional Overturning Circulation and the consequent shoaling of the local carbonate compensation depth. Here we present orbitally-resolved alkenone SST, surface productivity, IRD, and benthic foraminiferal isotope records from 2.6-3.6 million years ago (mya) in the North Pacific at ODP 883 (51°N, 167.5°E, water depth 2384m). This is the first dataset from the Pliocene North Pacific with a tuned oxygen isotope age model.

Our alkenone inferred SSTs are cooler (mean 2.9°C cooler over the study interval) than the Haug et al. (2005) record at nearby ODP 882 (50°N, 167°E, water depth 3244m), which appears to be due to a lesser influence of laterally transported alkenones from warmer regions at ODP 883. We also found that coccolithophore surface productivity in the North Pacific steadily decreased beginning 3.2 mya while diatom surface productivity remained relatively constant. IRD input to 883 begins at ~2.9 mya and the > 150µm IRD fraction maximum occurs at ~2.75 mya.

The orbitally-paced maxima in carbonate burial at ODP 882 reported by Haug et al. (2005) and Burles et al. (2017) are accompanied by maxima in Cibicidoides spp. δ13C at ODP 883, reflecting maxima in δ13C DIC. Cibicidoides spp. δ13C maxima are antiphase to variations in alkenone C37 total during the Mid Pliocene Warm Period, supporting their earlier conclusion that Pliocene carbonate deposition in the North Pacific was driven by deep convection and ventilation of North Pacific waters to a depth of at least 3255m. We also demonstrate that Cibicidoides spp. δ13C gradient between the equator and North Pacific was reduced during the Mid Pliocene Warm Period and M2 event compared to the Late Pleistocene. This reduced gradient implies that there was greater ventilation of the North Pacific in the Pliocene compared to the Late Pleistocene.