Lingwei Li, Boulder: Understanding Changes Using Geotracers

COMPASS 2024-10-02: Lingwei Li, University of Colorado Boulder
"Understanding Changes of Global Ocean Circulation Using Geotracers in Earth System Models"
Marine radiocarbon (14C) is widely used to trace deep ocean circulation, providing insight into the atmosphere-ocean exchange of CO2. During the Last Glacial Maximum (LGM), radiocarbon reconstructions suggest that the deep waters were poorly ventilated, trapping CO2 in the deep ocean and thus limiting the air-sea carbon exchange. In the first part of the presentation, I will share results from two transient climate model simulations with radiocarbon and ideal age tracers, and discuss what we can learn from these tracer-enabled simulations about the mechanisms of global ocean ventilation during the last deglaciation. Although simulated radiocarbon shows reduced ventilation at the LGM, the true model ventilation age is modestly younger mainly due to a stronger simulated glacial Antarctic Bottom Water (AABW) transport. Notably, the ocean ventilation age is considerably older around 14-12 ka than at the LGM because of a weakening of AABW transport. Our study suggests that, while 14C is the tracer most relevant to water ventilation ages, the inferred age can be different from the true age of a water mass. That means that, how radiocarbon should be used to infer ocean circulation changes, warrants further exploration. In the second part of the presentation, I will briefly introduce an ongoing project focusing on the sea ice-ocean exchange of microplastics in the Arctic, using a fully coupled Earth system model (CESM) with microplastic tracers.