Collection, analysis, and age-dating of sediment cores from coastal wetlands in Oyster Bay, Alabama, 2021

To assess historical environment changes associated with changes in hydrologic connection to the Gulf Intracoastal Waterway (GIW) and Mobile Bay, six sediment cores (50-80 cm in length) were collected in 2021 from two coastal wetland sites. These two sites in Gulf Shores, Alabama were: Oyster Bay Nature Preserve ("OB", a brackish marsh with natural hydrology) and the Emmet O. and Vina Wenzel Wetland Preserve ("WW", an impounded coastal wetland). The OB control site is a 1.5 square kilometer wetland with unobstructed natural oscillations in water levels and continuous connection to the GIW and Mobile Bay. The WW impounded study site is approximately 1.9 square kilometers; it has been cut off from saltwater exchange by three events. First, construction of the GIW in 1937 created berms of dredge spoils along the southern border. Second, culverts installed during completion of County Road 4 in the 1970s sharply reduced exchange between the wetland and the GIW. Third, Hurricane Ivan (2004) dropped a barge on the wetland; after several years, it was removed by dragging across the wetland and County Road 4 into the GIW. This sharply altered the geomorphology of the eastern section of the wetland located north of County Road 4. These three events have led to a nearly complete occlusion of the WW wetland from the GIW and Mobile Bay. As a result, the WW wetland does not experience tidal exchange and is continuously flooded due to retention of surface water from surrounding uplands. Transects were established, at both sites, starting from the tidal creek at OB and from the former tidal inlet at WW and moving toward the marsh interior. Cores were collected from three sites along each transect; core sections were age-dated using radioisotopes, and analyzed for carbon, nitrogen, and sulfur content and stable isotopes. Age-models were constructed from lead-210 in five of the six cores; the OB3 core (collected furthest from the tidal creek) was not dateable because the excess lead-210 profile was disturbed down to 20 centimeters. Accretion rates at OB were similar across the two dateable sites (OB1 and OB2) and through time with an average of 2.7 +/- 1.0 millimeters per year. Vertical accretion rates have increased with time across the WW sites, with the highest rates (6.0 +/- 1.2 millimeters per year) observed at WW2 (midway along the transect) since near total impoundment in 2008. Carbon accretion rates at OB1 and OB2 have remained similar through the past century at 65 +/- 28 grams Carbon per square meter per year. Prior to impoundment, accretion rates at WW were lower (38 +/- 24 grams Carbon per square meter per year) than OB. Since impoundment, carbon storage rates at WW have reached 132 +/- 60 grams Carbon per square meter per year.