A Last Glacial Maximum to Present Paleoenvironmental Investigation of Port Pegasus, Stewart Island, New Zealand

Almost all modern-day estuaries evolved during the Holocene and contain sedimentary sequences that preserve the evolution of these coastal environments. Incised-valley fill sequences in estuaries often contain facies that illustrate the transition from terrestrial to marine conditions during post-glacial sea-level rise. The timing of sea-level rise and inundation can be constrained by dating these transitions, and geochemical characterization of the facies exposed in sediment cores provides information about the ways in which sea-level incursion has altered the sediment deposition and carbon accumulation in these environments. Coastal margins are known to be hot spots of carbon accumulation and burial and important components of the global carbon cycle. However, more information is needed to fully understand carbon sequestration processes, rates, and variability, particularly as sea-level begins to rise again after the Holocene still stand (7.5-7.3 ka). The location of this study, Port Pegasus, is an incised valley estuary located on the southeast margin of Stewart Island, New Zealand, at 47°S. Multibeam bathymetric surveys in combination with tide and catchment data are used to characterize Port Pegasus as a drowned valley, mixed energy estuary with low fluvial input and sediment depocenters within its back basins, and seismic surveys are used to identify and interpret the stratigraphic fill as a transgressive sequence. Five sediment cores sample terrestrial to marine flooding surfaces and sedimentary depocenters and are analyzed for physical properties, paleomagnetic parameters, elemental data, and bulk carbon and nitrogen concentrations and stable isotopes. Radiocarbon ages date the terrestrial to marine transition, identified using geochemical data, and the flooding of Port Pegasus at -22 m to 11.6-11.1 ka. Carbon accumulation rates show that zones of high carbon accumulation shifted from a terrestrial depocenter in the central basin of the inlet with an accumulation rate of 10 g C m-2 yr-1 to the present-day marine depocenters in the isolated back basins with an average rate of 19 g C m-2 yr-1, suggesting that sea-level incursion had a significant influence on carbon accumulation dynamics in the estuary. The source of the organic carbon has also changed; the underlying facies shows a clear abundance of terrestrial organic matter, whereas all Holocene and modern sediment is dominated by marine organic matter. Analysis of back basin sediment cores demonstrates that Port Pegasus may be a promising new location for paleoclimate reconstructions of Southern Hemisphere westerly wind variability and that cores extracted closer to the fluvial inputs in the estuary will likely contain terrestrial signals that vary with the magnitude of freshwater flux. This study gives insights into New Zealand Holocene sea-level rise and the potential role of meltwater pulses in coastal estuarine formation as well as the significant effect of sea-level transgression on coastal carbon storage dynamics and the important role that drowned valley systems such as Port Pegasus play in the global carbon cycle