Export of terrigenous dissolved organic matter in a broad continental shelf

Abstract

Export of terrigenous dissolved organic matter (DOM) from rivers to the ocean plays an important role in the carbon cycle. Observations from six research cruises in 2014 were used to characterize the seasonal evolution of terrigenous DOM in the shallow and broad South Atlantic Bight (SAB) shelf. While DOM with a strong terrigenous molecular, optical and isotopic signature was restricted to a coastal band early in the year, a plume with terrigenous DOM extended further to the shelf break in late spring. The offshore transport of this terrigenous DOM was consistent with wind‐driven advection in a surface Ekman layer. On time scales spanning about 1 month, the traceable riverine DOM compounds were mostly resistant to bio‐ and photo‐degradation, and the decrease in their relative abundance over the shelf following peak river discharge during spring was consistent with dilution of the river plume due to entrainment of oceanic water associated with wind‐driven mixing. Comparisons between optical absorbance measurements and ultrahigh resolution mass spectrometry data revealed that the fraction of the DOM pool with a riverine signature in the SAB can be estimated using the spectral slope coefficient of chromophoric DOM in the 275–295 nm range. This finding opens up the possibility of observing the distribution of riverine DOM on the SAB shelf in high spatial resolution and by using remote sensing methods, a crucial step for quantifying shelf‐slope exchange and the fate of terrigenous DOM in shelf seas.

Dissolved Organic Matter Composition in a Marsh‐Dominated Estuary: Response to Seasonal Forcing and to the Passage of a Hurricane

“…The time series was not correlated with river discharge or with precipitation. The pattern revealed by the loading of PC 1, shown color coded in a van Krevelen diagram (Figure b), was different from the pattern typically observed in river to ocean transects (Medeiros, Babcock‐Adams, et al, ; Medeiros, Seidel, Dittmar, et al, ; Medeiros, Seidel, Gifford, et al, ; Medeiros, Seidel, Ward, et al, ; see also Figure d). This suggests that the mode was not related to the varying contribution of terrigenous vs marine sources to the estuarine DOM pool.…”

“…For the current sampling period, the total DOC export was larger at 108 Gg C/year. Medeiros, Babcock‐Adams, et al () showed that DOC flux is highly correlated to discharge at the Altamaha River. Thus, the 50% increase in DOC export compared to the long‐term average is likely related to increased river discharge during the study period associated with El Niño conditions (Sheldon & Burd, ).…”

“…The Altamaha River discharge is characterized by strong seasonality, generally peaking in March or April and reaching a seasonal minimum during fall (Medeiros, Babcock-Adams, et al, 2017). The peak in discharge in 2016 occurred a few months earlier in January and February (Figure 2a), however, due to increased rainfall associated with the positive phase of the El Niño-Southern Oscillation (Hansen et al, 1997;Keener et al, 2010).…”

Black Sea dissolved organic matter dynamics: Insights from optical analyses

“…The terrigenous content of DOM can be estimated from optical properties such as S 275–295 , based on its relationship with the terrigenous biomarker lignin, which has been employed as a proxy for such material in highly river‐influenced systems (Fichot and Benner ; Fichot et al ; Medeiros et al ). Assuming the previously identified S 275–295 ‐lignin relationships are valid for the Black Sea, S 275–295 values reported here suggest that Black Sea DOM is ∼ 4–12% terrigenous, having the lowest percentages at the surface and highest in the BBL, which is counterintuitive.…”

Chemical Speciation of Copper in a Salt Marsh Estuary and Bioavailability to Thaumarchaeota

“…This suggests a midestuarine source of L 2 and HS Cu , which could be from increased fluxes of sedimentary organic carbon into the estuary, potentially partly linked to higher bioturbation by fiddler crabs and other invertebrates during summer. Published records show DOC concentrations range from around 3.0 to 9.0 mg/L along the estuary and are not conservative with salinity (Hodson, 2005;Medeiros et al, 2017). Like L 2 , the concentration of humic substances was generally highest in spring and generally constant from September to December, with elevated concentrations relative to salinity in August.…”