Abstract
The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.
Highlights
The age of terrestrial organic components in marine sediments reflects their residence times in continental reservoirs
This core captures the “bomb spike” derived from thermonuclear weapons testing, which was most active above ground in the 1950s and early 1960s
Bulk geochemical data imply that the G-B rivers supply the fine-grained sediments deposited at the head of the canyon, and the sediment source was constant over the sampling interval (Supplemental Table S1)[14,15]
Summary
The age of terrestrial organic components in marine sediments reflects their residence times in continental reservoirs. While previous work has identified several components of the bulk OC possessing different sources and average ages within the G-B watershed[3] (e.g., petrogenic C, bulk biospheric C, etc.), the age spectrum of the terrestrial OC, and leaf-wax fatty acids, delivered to the Bengal Fan has not been fully resolved and quantified. An 18 m-long piston sediment core (SO188–336KL) was collected in 2006 at the head of the canyon that bisects the Bengal shelf[13] (Supplementary Fig. S1). This core spans 1945 to 2006 CE according to age models derived from 137Cs records and correlation of storm-derived deposits with known cyclonic events Bulk OC is systematically more 14C-depleted than the corresponding fatty acids
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