Tracing past ocean gateways with the marine nitrogen cycle

The ocean modulates the response of Earth’s climate to forcing by transporting and storing large quantities of heat and carbon. Because the ocean’s transport pathways are dependent upon the position of the continents, it is thought that past changes in ocean gateways may have had an outsize impact on Earth’s climate evolution. However, pinning the timing of ocean gateway change is notoriously difficult, as paleoproxies of ocean transport either primarily reflect deep water exchange and are insensitive to shallow gateways (e.g., neodymium isotopes) or are confounded by processes unrelated to ocean transport (e.g., hydrological changes for oxygen isotopes). As a consequence of this uncertainty, significant debate exists over whether changes in ocean gateways drove—or were wholly independent from—major climate changes over the Cenozoic.

Here I will present a new tool for studying past ocean gateways grounded in the marine nitrogen cycle. Regional imbalances in ocean fixed nitrogen inputs and losses lead to distinct nitrate isotopic compositions in the shallow subsurface ocean. I reconstruct these signatures in the past using the N isotopic composition of organic matter preserved within fossil foraminifera in two case studies: (1) the shoaling of the Central American Seaway, which connected the tropical Pacific and Atlantic Oceans until the Pliocene, and (2) the late Pleistocene sea-level history of the Bering Strait, which links the North Pacific and Arctic Oceans today. Implications of these reconstructions for the relationships between ocean gateways and Earth’s climate change will be discussed.

About this Series

The Atmosphere, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmosphere, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm in 54-915. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.

Contact: sacklunch-committee@mit.edu for Zoom information