ORGANIC MATTER STORAGE IN ABANDONED CHANNEL DEPOSITS IN CONGAREE NATIONAL PARK, SOUTH CAROLINA

Organic matter stored in floodplain sediments represents an important and measureable component of the global carbon cycle. The Congaree River floodplain, located in the coastal plain of South Carolina, holds a potentially large stock of organic matter (and carbon) in abandoned channel deposits. The purpose of this study is to quantify organic matter storage within these abandoned channel deposits using loss-on-ignition (LOI) techniques. Five cores were collected from three abandoned river channels using a Macaulay peat sampler. Cores, which ranged in depth from 2 to 4 m, were sub-sampled at 15 cm intervals of known volumes. A total of 107 sub-samples were dried, weighed, analyzed for bulk density, and analyzed for percent dry organic matter (DOM) using LOI techniques. Several LOI techniques were found in the literature, but these yielded variable results related to incomplete combustion or complications from thermal dehydration of clay minerals. Based on multiple analyses of field samples and kaolinite check standards, combustion at 440ºC for 12 hours resulted in the most effective removal of DOM without dehydrating clay minerals. Bulk density averaged 0.69 +/- 0.40 g/cm³ and ranged from 0.15 to 1.55 g/cm³. Weight percent DOM averaged 19.34 +/- 14.40 and ranged from 3.36 to 56.17. Bulk density shows an inverse, curvilinear relationship with weight percent DOM (R² = 0.88). Total DOM, calculated as weight percent DOM x bulk density, averaged 0.08 +/- 0.02 g/cm³ and ranged from 0.04 to 0.16 g/cm³. Average DOM per core ranged from 0.05 to 0.09 g/cm³. All cores demonstrated clear soil horizons in the top 30 cm. Cores Weston Lake 2 and Butterfly Pond were both dense and clay-rich, with relatively little down-core variation. Cores Weston Lake 1, Wise Lake 1, and Wise Lake 2 show an upper, organic-rich zone (with some sub-zones) overlying a deeper, homogeneous clay-rich zone. Down-core variations appear related to relative core position within the abandoned channel as well as channel position within the floodplain. The LOI methods calibrated here can be used to compare additional samples from abandoned channels and other floodplain depositional environments. Results can be extrapolated to constrain total volumetric organic matter (and carbon) storage in abandoned channel deposits across the Congaree River floodplain and beyond.