Interactions between iron and organic carbon in a sandy beach subterranean estuary

Abstract

Understanding the behavior of terrestrially derived dissolved organic carbon (DOC) through subterranean estuaries (STEs) is essential for determining the carbon budget in coastal waters.However, few studies exist on the interaction of organic carbon (OC) and iron (Fe) in these dynamic systems, where fresh groundwater mixes with recirculated seawater. Here, we focused on the origin and behavior of DOC, and we quantified the relative proportion of OC trapped by reactive Fe-hydroxides along a sandy beach STE. The  13 C-DOC signal in beach groundwater seems to respond rapidly to OC inputs. Our results show a terrestrial imprint from the aquifer matrix dominated by the degradation of particulate organic carbon (POC) issue from an old soil horizon composed of terrestrial plant detritus ( 14 C dating ~800 to 700 years B.P) which is buried below the Holocene sand. Even though the system can be sporadically affected by massive inputs of marine OC, this transient marine imprint seems to be rapidly evacuated from the STE. As reported in soil and in marine mud, Fe-OC trapping occurs in the sandy sediment of the STE. At the groundwater-seawater interface, newly precipitated reactive Fe-hydroxides interact with and trap terrestrial OC independently of the DOC origin in beach groundwater. The molecular fractionation of DOC along the STE and preferential trapping of terrestrial compounds favor the in situ degradation and/or export of non-Fe-stabilized marine-derived molecules to coastal waters. These findings support the idea that the sandy beach STE acts as a transient sink for terrestrial OC at the land-sea interface and contributes to the regulation of marine vs. terrestrial carbon exports to coastal waters.

Reactive Iron and Iron‐Bound Organic Carbon in Surface Sediments of the River‐Dominated Bohai Sea (China) Versus the Southern Yellow Sea

“…Both δ 13 C bulk and δ 13 C non-Fe-OC in the BHS sediments (Table 3) are within a narrow range that is commonly reported for bulk OC in marine sediments (Burdige, 2006), whereas the range of calculated δ 13 C Fe-OC is much wider than that of the 13 C bulk and δ 13 C non-Fe-OC . Large variations of δ 13 C Fe-OC were also reported in previous studies (Lalonde et al, 2012;Ma et al, 2018;Salvadó et al, 2015;Sirois et al, 2018;Q. Zhao, Poulson, et al, 2016).…”

“…The results are distinctly different to those in the SYS, where Fe-OC is consistently 13 C-enriched relative to 13 C non-Fe-OC at all sites, with a Δδ 13 C average of 7.60 ± 6.98‰ (Table 3). In fact, both 13 C Fe-OC enrichment and depletion relative to 13 C non-Fe-OC and/or 13 C bulk have been reported for marine sediments (Lalonde et al, 2012;Salvadó et al, 2015;Shields et al, 2016;Sirois et al, 2018) and forest soils as well (Q. Zhao, Adhikari, et al, 2016).…”

“…Fe-bound OC (Fe-OC) is reported to account for 21.7 ± 7.8%, on average, of total OC (TOC) in global continental shelf sediments underlying oxic waters (Lalonde et al, 2012). Further studies on Eurasian Arctic shelf (Salvadó et al, 2015), river delta (Shields et al, 2016), extensive continental shelves of China (East China Sea [ECS] and southern Yellow Sea [SYS]; Ma et al, 2018), a subterranean estuary (Sirois et al, 2018), and boreal lakes (Peter & Sobek, 2018) indicate that percent fractions of Fe-OC to TOC (i.e., f Fe-OC ) are in a wide range from nearly 0% to as high as 42%. Several factors have been invoked to explain the wide variations of f Fe-OC , including binding mechanisms of OC to Fe oxides (surface adsorption versus coprecipitation), mineralogy and reactivity of Fe oxides, and redox cycling of Fe, but no consensus has as yet been reached (Kleber et al, 2015).…”

The Role of Reactive Iron in the Preservation of Terrestrial Organic Carbon in Estuarine Sediments

“…Alternatively, the higher proportion of hematite in the mobile muds could be due to the maturation of Fe oxides that experience long‐term resuspension and redeposition cycles before eventual burial (Raiswell, ; Stucki et al, ). Recently, Sirois et al () proposed that oxidation‐reduction oscillations likely enhance Fe R binding with terrestrial‐derived DOC; however, they also argued that the OC‐Fe association is likely a transient sink for OC—due to microbial reduction of iron. Our results supported this hypothesis that frequent physical reworking coupled with rapid iron redox cycling in mobile muds is likely removing part of Fe‐OC.…”

Groundwater as a source of dissolved organic matter to coastal waters: Insights from radon and CDOM observations in 12 shallow coastal systems

“…While it is now clear that groundwater can be a major source of dissolved nitrogen to local and global coastal systems (Slomp and Van Cappellen 2004;Cho et al 2018), groundwater discharge has generally been overlooked in coastal carbon budgets (Moore 2010;Santos et al 2015). However, concentrations of dissolved organic carbon (DOC) are usually greater in groundwater than coastal surface coastal waters in spite of high variability driven by residence times, redox conditions, and carbon sources and transformations (Kim et al 2012;Seidel et al 2014;Linkhorst et al 2017;Sirois et al 2018). Local estimates of groundwaterderived DOC to coastal waters have ranged from 20% to over 100% of nearby river inputs (Goni and Gardner 2003;Stewart et al 2015;Sadat-Noori et al 2016).…”