Morphological Variability of the Deep Dwelling Foraminifer Globorotaloides hexagonus: A clue to changing conditions within the Oxygen Minimum Zone?

The shells of planktic foraminifera found in marine sediments are one of the most often used proxies for conditions in the overlying ocean. However, as most species are confined to the shallow ocean, they are less suited for interpreting environmental changes in critical deep pelagic environments such as the Oxygen Minimum Zone. Recently, depth stratified plankton tows from the Eastern Tropical North Pacific have confirmed that the foraminifera species Globorotaloides hexagonus is living within the Oxygen Minimum Zone and that calcification intensity, as assessed from individual shell area-normalized weight, is lower in individuals recovered from lower oxygenation levels. This raises the possibility that changes in area-normalized weight of G. hexagonus could offer clues to the how mid-water Oxygen Minimum Zone environments have varied through time. We present records of area-normalized weight from modern G. hexagonus as well as from two cores spanning from the Last Glacial Maximum to the recent Holocene along the Mexican Margin (MAZ1-E04; 0-36 ka; 1,463 m depth) and the Panama Basin (TR163-23; 2-25 ka; 2,730 m depth), where the depth and intensity of the Oxygen Minimum Zone is thought to have evolved over the last deglaciation. In the Panama Basin, cored below the Oxygen Minimum Zone, area-normalized weights are overall higher through the last glacial interval than during the deglaciation. On the Mexican Margin, where the Oxygen Minimum Zone impinges upon the continental shelf, area-normalized weight can be compared to previously published proxy records for benthic oxygenation at the site. These records represent the first step in probing the viability of area-normalized weight variability in G. hexagonus as an indicator of conditions within the Pacific Oxygen Minimum Zone on millennial timescales.