Novel analytical strategies for tracing the organic carbon cycle in marine and riverine particles

Abstract

Particulate organic carbon (POC) in the ocean and mobilized by rivers on land transfers ~0.1% of global primary productivity to the deep ocean sediments. This small fraction regulates the long-term carbon cycle by removing carbon dioxide from the atmosphere for centuries to millennia. This thesis investigates mechanisms of POC transfer to the deep ocean by analyzing particles collected in transit through two globally significant carbon reservoirs: the Southern Ocean and the Amazon River Basin. These endeavors test the hypothesis that organic matter composition controls the recycling and transfer efficiency of POC to the deep ocean, and illustrate new applications for ramped pyrolysis/oxidation (RPO), a growing method of POC characterization by thermal stability. By coupling RPO to stable and radiocarbon isotope analyses of riverine POC, I quantify three thermally distinct soil organic carbon pools mobilized by the Amazon River, and evaluate the degradability and fate of these different pools during transport to the coastal Atlantic Ocean. More directly, RPO analyses of marine samples suggest that POC transfer in the water column is in fact selective. Observations of consistent biomolecular changes that accompany transport of phytoplankton-derived organic matter to depth across the Southern Ocean support the argument for preferential degradation of specific POC pools in the water column. Combining discussions of POC recycling and transfer across both marine and terrestrial systems offer new perspectives of thermal stability as a proxy for diagenetic stability and POC degradation state. The challenges of interpreting RPO data in these two environments set the stage for applying the technique to more controlled experiments that trace POC from source to long-term sink.

Mineral protection regulates long-term global preservation of natural organic carbon

“…First, all samples that were not acidified prior to RPO analysis were screened for carbonates. This includes PPL-extracted and ultrafiltered DOC 33 , Amazon River suspended sediments 39 , and a subset of soils…”

“…33,56 . Because our dataset contains samples analyzed using multiple methods, we assessed the influence of sample pre-treatment and RPO instrumental conditions on Specifically, there are three methodological variables that might influence OC thermal stability within our dataset: (i) whether or not a sample was acidified38,39 , (ii) the type of acid treatment, and (iii) whether the RPO instrument was operated in pyrolysis or oxidation mode.…”

“…Because DOC cannot contain carbonates by definition, all DOC samples were retained. For Amazon River sediments, Rosengard39 analyzed one sample OC would likely influence RPO results. This resulted in the exclusion of only a single Canadian soil sample collected from an agricultural plot38 .Third, samples were screened for OCpetro contribution.…”

Influence of Different Acid Treatments on the Radiocarbon Content Spectrum of Sedimentary Organic Matter Determined by RPO/Accelerator Mass Spectrometry

“…We did not make 14 C measurements on the high temperature fraction of these samples. In high carbonate samples, IC may start to decompose at lower temperatures than expected based on thermodynamics due to non-first-order decay (Rosengard 2017; Hemingway et al 2017b). The δ 13 C results suggest that IC can be present in even the lowest temperature interval in some instances (T 1 , Berm) (Table 3).…”