USING IMAGE ANALYSIS TO ESTIMATE GRAIN-SIZE DISTRIBUTIONS OF SAND FRACTION OF OLIGOCENE NEWFOUNDLAND RIDGE CONTOURITE DRIFT SEDIMENTS, IODP SITE U1411

Thermohaline circulation is a critical process that transfers heat and nutrients throughout Earth’s oceans via large-scale currents. It is important to better understand how these currents were affected by climate change in the past in order to improve predictions of ocean circulation in the future. The Deep Western Boundary Current (DWBC) is the deep-ocean current in the North Atlantic that brings cold, dense water formed at high northern latitudes southward. This study aims to provide insight into the response of the DWBC to climate changes at the Eocene-Oligocene transition (EOT) by analyzing grain size characteristics of the terrigenous coarse fraction (>63 μm) of Eocene-Oligocene age sediments from the Newfoundland Ridge Drift Complex, IODP site U1411, which is interpreted to have formed under the influence of the DWBC. Understanding variations in these terrigenous sands over time will allow us to understand how current speed changed in response to a changing climate. We used the open-source Digital Grain Size Project (https://github.com/dbuscombe-usgs/DGS) to analyze high-resolution images of these sands to determine grain-size distributions for 52 samples spanning a depth range of 25-147 m below the sea floor and 24-32 Ma.

Information including mean, median, and sorting of U1411 sands was collected. Due to only trace amounts of sand present in Eocene samples, grain-size analysis was not collected for the Eocene-Oligocene Transition (~34 Ma) time interval. Mean and median grain size remained relatively constant throughout the entire study interval, with means ranging from 87-121 μm and medians ranging from 73-120 μm. All samples are considered very well sorted, and this also did not change significantly over the entire interval. The consistency of these metrics is in contrast to the previously generated mass-fraction data of these same sands, which shows a distinct increase in sand content beginning at the earliest Oligocene and persisting through the rest of the study interval. This shows that while the current was carrying and depositing more sand, the size and sorting characteristics of this sand was not changing. This may suggest a constant sediment source during this time, but future work investigating provenance may help to understand what this means regarding current dynamics and sediment source.