Title: The influence of size (and composition) on the biogeochemical cycling of marine organic matter
Abstract: Marine organic matter (OM) represents one of Earth’s largest actively cycling reservoirs of organic carbon and nitrogen. Most OM is fixed during primary production and is rapidly cycled and remineralized. This process leaves behind many dissolved and particulate OM (DOM, POM) forms of varying biological reactivity – together comprising two vast reservoirs of nonliving organic matter in seawater. The processes controlling OM production and removal are important for carbon and nitrogen biogeochemical cycles, which regulate climate. However, many possible cycling mechanisms have hindered our ability to quantify OM transformation, degradation and turnover rates in the ocean. The size-reactivity continuum model presents a conceptual framework linking the biological and microbial carbon pumps (MCP) for understanding the mechanisms governing OM formation and mineralization. Based on early incubation experiments, the size-reactivity model suggests larger OM size classes are more bioavailable and more rapidly remineralized by microbes than smaller OM size classes.
In this talk, I will highlight several of our radiocarbon (14C) studies which expand upon the size-reactivity model – seeking to understand the explicit relationships between OM size, 14C age and chemical composition. An analysis of OM carbon:nitrogen (C:N) ratio and 14C age spanning size from large POM to small DOM reveals that OM size is negatively correlated with 14C age and C:N ratios in the coastal, surface and deep Pacific Ocean. Our measurements suggest that OM is increasingly chemically-degraded as it decreases in size, and that small POM and DOM persist in the ocean longer than their larger counterparts. Based on these correlations, we estimate MCP production rates of small, biologically recalcitrant DOM at 0.11-0.14 Gt C yr-1 and ~0.005 Gt N yr-1 in the deep ocean. Our results suggest that the preferential remineralization of large over small POM and DOM is a key process governing OM cycling and deep ocean C storage. Taking a closer look at the DOM pool, we use a series of increasing DOM molecular size fractions from three depths in the Pacific to show that DOM 14C ages also decrease with increasing size. A size-age distribution model generally predicts the concentration and 14C age of size-fractionated DOM in the surface ocean, but reveals more complexity in the deep ocean. Ultimately these studies, together with past molecular-level and incubation work, suggest that biodegradation processes not only shape the size distributions of OM, but also their chemical composition, reactivity and storage in the deep ocean.