Tracing Pliocene ocean circulation using seawater δ18Ow reconstructed from Δ47 of marine mollusk fossils

A wealth of knowledge about climate, hydrology, atmospheric science and more can be accessed by studying the oxygen isotopic composition of modern waters. Unfortunately, modern studies can only reveal current behavior of the earth system and it is much more challenging to measure this powerful isotopic tracer in past waters as the waters themselves generally no longer exist. Poor knowledge of past δ18Ow is especially problematic for paleoclimate studies based on δ18O of fossil carbonates, which require an assumption about δ18Ow to reconstruct temperature. Incorrect choice of δ18Ow can bias temperature estimates in either direction by ~5°C per 1‰ error in δ18Ow. The clumped isotope paleothermometer (Δ47) can tease apart the influences of temperature and δ18Ow on carbonate δ18O, yet many Δ47 studies focus only on the temperature data, leaving a potentially valuable source of information on the table. Here, I turn the spotlight onto δ18Ow. I will show how Δ47-derived-δ18Ow tracks the location of coastal currents in North Carolina and Virginia in the Pliocene. Today, the cooler, fresher, δ18Ow-depleted southerly Virginia Coastal Current meets the warmer, saltier, δ18Ow-enriched northerly Carolina Coastal Current at Cape Hatteras, NC, where the Gulf Stream breaks off eastward into the Atlantic Ocean. Δ47 analysis of Pliocene fossil scallops from this region supports the past penetration of the Virginia Coastal Current further south via both temperature and Δ47-derived-δ18Ow data, potentially due to movement of the coastline under higher sea levels. This study serves to demonstrate the power of Δ47-derived-δ18Ow to answer questions about ocean circulation in the past, and emphasizes the need for greater spatial and temporal coverage of seawater δ18Ow data in today's oceans to facilitate paleo-interpretations, especially in coastal areas where mollusks live.