NOAA/WDS Paleoclimatology - North Pacific Thorium Isotopes over the last 500,000 years

Evaluating the stratigraphy of the North Pacific is the first step towards understanding the role of this region in climate variability in the past. This region has been notoriously difficult to work in due to the presumed low sedimentation rates (<0.1 cm/kyr), which would likely be inadequate to capture glacial-interglacial cycles of the Late Pleistocene (∼500,000 years). These low sedimentation rates have been estimated from magnetostratigraphy and biostratigraphy, but those techniques are only minimally constrained during this geologically recent time period. Here, we reconstruct average sedimentation rates for the last 500 kyr across the deep (>3000 m) North Pacific using 230Th depth profiling, which is based on the exponential radioactive decay of 230Th and is one of only a few chronological tools applicable to sediment within this relatively calcium carbonate deficient region. Sedimentation rates are indeed low (<0.25 cm/kyr) within the subtropical gyre, particularly between 20-30˚N, but sedimentation rates greater than 0.5 cm/kyr are observed between 30-40˚N, particularly west of 200˚E. Sedimentation rates are higher underneath the Asian dust plume, and no water-depth dependence is observed below 4000 m. Sedimentation rates from 230Th depth profiling generally agree with other methods, but they may be more representative of the average sedimentation rates for the last 500 kyr compared to other proxies that operate on much shorter (e.g., radiocarbon) or much longer (e.g., magnetostratigraphy) timescales. Overall, the efficacy of 230Th chronology is a promising tool for further investigation into the history of the North Pacific in the past.