A modern perspective on ancient oceans

Abstract:

Proxy-derived sea surface temperature (SST) estimates form the groundwork of our understanding of past climates. However, unlike the modern well-instrument world, ancient oceans are comparatively under-sampled and most proxy data come from just two environments: epeiric seas and continental margins. In order to accurately reconstruct paleoclimate metrics, such as latitudinal temperature gradients, we must therefore understand how SSTs from these environments fit into the broader global climate. Here, we use modern SSTs to demonstrate that the environments from which most paleoclimatic data are drawn are systematically offset from zonal mean temperatures. Modern epeiric sea analogs are both warmer and more seasonal than open-ocean values from the same latitudes; as most Paleozoic SSTs come from epeiric seas this suggests that the warm temperatures inferred from Paleozoic proxy data are due in part to a geographic sampling bias. Similarly, continental margins exhibit consistent and predictable annual and seasonal deviations related to gyre circulation. We use a pseudoproxy analysis to demonstrates the ease with which latitudinal temperature gradients can be under- or overestimated simply by under- or over-sampling the dynamical regimes associated with gyre circulation and compare modern SSTs with several earth system model simulations to assess the extent to which they are able to reproduce these observed patterns. Collectively, our findings highlight the important paleoclimate implications of geographical sampling biases. However, capitalizing on our understanding of modern dynamical processes provides a promising new avenue whereby global proxy compilations, when viewed holistically, can provide insight into ancient ocean dynamics and greater clarity about past thermal gradients.