62nd Annual Report on Research 2017

The scientific objective of this study is to quantify relationships between dissolved organic carbon (DOC) molecular-level composition (via ¹H-NMR spectroscopy) and radiocarbon (¹⁴C) age in several marine environments. In particular, we are studying the formation, diagenesis and removal mechanisms of recalcitrant DOC (i.e. carboxylic-rich alicyclic molecules; CRAM). Recalcitrant DOC can also be absorbed and transported by sinking particles and preserved in marine sediments. This dissolved protokerogen can influence the oceanic kerogen reservoir on geological timescales.

To date, we have accomplished approximately two-thirds of the goals that we proposed. We have measured all of the seawater samples from the North Atlantic and the Gulf of Mexico for DOC ∆¹⁴C and d¹³C signatures and published most of these results (see publication list). We have finished developing the ¹H-NMR method at UCI on seawater samples and have analyzed approximately half of the proposed samples using this technique. We estimate to have completed all ¹H-NMR seawater analysis in 2018. Details of our progress in year-2, and plans for year-3 appear below.

We have measured ∆¹⁴C and d¹³C of water samples from the Mississippi River, river plume/shelf, slope and offshore Loop Current sites in the Gulf of Mexico (Walker et al. 2017, GRL). We find that DOC ∆¹⁴C values decrease with distance from the river mouth, and that there are similar ∆¹⁴C values found in the deep Gulf of Mexico and the deep Carribean Sea. In addition, we found an isotopic imprint of mid-water column methane oxidation still persisting in the Gulf of Mexico following the Deepwater Horizon oil spill.

Dr. Walker is analyzing seawater samples using ¹H-NMR spectroscopy to obtain an understanding of how the molecular structure of DOC changes in its transfer from riverine, to estuarine, to open ocean pools. We have refined the water suppression method at UCI to obtain adequate spectra for our seawater samples. We found that storage of samples >24hrs in NMR tubes allowed for aromatic contamination from the caps to “grow into” our spectra. We also learned that additional 0.2µm filtration of seawater samples, thought to be a requirement for high particulate samples, resulted in gross contamination. The insuppressable water peaks found in samples of these types was resolved by “sipping” carefully the top of our stored samples and not shaking them at all. To date ~50% of the Gulf of Mexico spectra have been analyzed. These spectra will be presented at the upcoming 2018 Ocean Sciences Meeting in Portland, OR. We are very excited about the new approach that this opportunity provides for our research on refractory DOC (protokerogen) in riverine and oceanic systems of the Gulf of Mexico and the North Atlantic.

This study will help answer fundamental questions involving the production, diagenesis and preservation of marine DOC. In particular, we are focusing on understanding the formation and removal mechanisms of recalcitrant DOC (RDOC). RDOC is a contributor of pre-aged, organic matter to sinking particulate organic matter. In this way, RDOC is an important source of protokerogen preserved in marine sediments over geologic timescales.

Over 85% of the proposed funding is being used specifically for the advanced education of one doctoral student (Christian Lewis) and one undergraduate research assistant (Noreen Garcia). Christian Lewis is learning how to conduct DOC isotopic analyses and molecular level characterizations of our seawater samples. Mr. Lewis and Dr. Walker have collected more samples on a cruise in the South Pacific in fall 2016, which included a new solid phase extraction technique that are valuable for extracting protokerogen-like material from seawater. DOC extracted using this technique is to be subjected to molecular level characterization using ¹H-NMR and incorporated into the results obtained from the ACS PRF study.

Finally, a select subset of these total seawater samples have been measured inside and outside of the oxygen minimum zone of the Eastern Tropical North Pacific. These ¹H-NMR spectra suggest large compositional changes within the ocean water column concomitant with a succession of microbial metabolism (i.e. oxic heterotrophy, to denitrification, and anerobic ammonia oxidation). We are starting to prepare another publication funded by this ACS PRF study.