Terrestrial Organic Matter in the Vancouver Island Fjords:
mixed methods * Methods development*Ocean sciences
Overview
Understanding how carbon cycles between the land and ocean is important for determining how climate and environmental changes have occurred in the past and how they will occur in the future. However, quantifying these processes relies on using proxies for organic carbon to reconstruct the carbon cycle through time. This study evaluated a proxy for terrestrial organic carbon, the Branched and Iseprenoid Tetraether (BIT) Index, and compared its performance to other proxies in order to determine how to develop the most robust picture of carbon cycling in the coastal ocean.
The coast of Vancouver Island is indented by a series of temperate fjords. Terrestrial carbon enters fjords from rivers at the head of each fjord, as well as from non–point source debris inputs from the sides of the fjords. Temperate fjords are thought to potentially trap significant amounts of carbon as a result of the high input of refractory (terrestrial) OC. Nuwer and Keil (2005) demonstrated the importance of fjords in storing terrestrial OC, estimating that fjord sediments may be responsible for trapping at least 12% of the total OC buried in continental margins over the last 100,000 years. This project used multiple proxies to detangle sources of organic carbon in the Vancouver Island Fjords.
Role: Oceanographic Researcher
Question
What are the sources and quantity of organic carbon buried in the Vancouver Island fjords' surface sediments as determined by multiple proxies, including the BIT Index, isotopic, and lignin analyses? What are the implications for using the BIT Index to reconstruct terrestrial organic matter in the ocean over time?
Analytic Approach
Surface sediments were analyzed from the Washington–Vancouver Island Margin, two Vancouver Island fjords, and Puget Sound. Margin sediment was obtained using a multicorer during a summer 2004 cruise aboard the RV Wecoma. All cores were sliced into 0.5–2-cm sections immediately after sampling, and the 2–3-cm interval was used for lipid, lignin, and bulk geochemical analyses. Vancouver Island fjord and Puget Sound sediments were collected using a Van Veen grab (approximately the top 20 cm) during a cruise aboard the RV Clifford A. Barnes in the summer of 2005 (Vancouver Island) and the aboard the RV Thomas G. Thompson in spring 2006 (Puget Sound).
Samples underwent elemental and isotopic analysis, lignin analysis and the BIT Index was calculated. The results were analyzed spatially using GIS and compared to each other.
Key Findings
A significant difference was found between the amount of carbon found to be terrestrial calculated when using isotopic or lignin analysis versus when using the BIT Index, as shown in the figure below. This implies that the BIT Index, which derives from lipids in microbial membranes, may be more indicative of soil organic carbon rather than all terrestrial organic carbon. This work has influenced how the BIT Index is used, as it is now viewed as a soil proxy rather than a total organic carbon proxy.
