Coastal deltaic floodplains are responsible for 40-50% of global coastal and marine carbon (C) burial and yet are often excluded from blue carbon literature. The Wax Lake Delta (WLD) is an unplanned Atchafalaya bayhead delta formation resulting from the dredging of the Wax Lake Outlet in 1942 to reduce downstream flooding of Morgan City. Twelve 4-5 m Vibracores were taken throughout the delta chronosequence to investigate ecological succession and C storage during the entirety of WLD’s development. An oyster shell bed that can be found throughout the delta delineates the beginning of the WLD facies in 1952.
Results/Conclusions
We identified five distinct layers within the cores of organic-rich, distributary mouth bar, sandy-channel fill, abandoned-channel fill, and prodelta deposits. As a young active delta its sediments are dominated by fine sand but the upper 0.4-0.7 m demonstrate a distinct transition to increased organic matter inputs. This organic-rich layer and the prodelta deposits have the highest C densities (11±1.7 and 9.6±1.7 mg cm-3, respectively) compared to the fine sand-rich facies (5.5-6.7 mg cm-3). In the organic-rich layer >90% of the C is organic compared to 30-53% in the lower facies. Furthermore, there is a significant difference within the sand-rich facies between what was deposited on a mouth bar and a channel (p<0.05). Spikes in organic matter and C content correspond to decreases in bulk density throughout the record. The lower bulk density layers formed during both low flood periods and once the WLD became subaerial in ~1973 when vegetation colonization was prevalent. We are using bay topography, chronosequence position and hydrogeomorphology to estimate the total C stock of the delta. With over 40 years of continuous subaerial land building, WLD provides a stark contrast to most of Louisiana’s retrograding coastline and is considered a model for future sediment diversions and provides an example of the great importance of coastal deltas in global C burial.