Date of Award
Doctor of Philosophy (PhD)
Earth & Environmental Science
Benjamin P. Horton
Obtaining a record of present and past storms, floods and sea levels, and their extent of geological and ecological impacts, is one means to assess future risk, reveal the spatial and temporal variability of coastal inundation and decipher its relationship with global climatic changes.
In the deltaic wetlands of Louisiana, I tracked sedimentation from the historic A.D. 2011 Mississippi River flood. The sediment deposited in wetlands during the 2011 flood was distinguished from earlier deposits based on biological characteristics, primarily absence of plant roots and increased presence of centric (planktonic) diatoms indicative of riverine origin. By comparison, the lithological (bulk density, organic matter content, and grain size) and chemical (stable carbon isotopes of bulk organic matter) properties of flood sediments were nearly identical to the underlying deposit. Flood sediment deposition was greatest in wetlands near the Atchafalaya and Mississippi Rivers and accounted for a substantial portion (37% to 85%) of the annual accretion measured at nearby monitoring stations. The amount of sediment delivered to those basins (1.1-1.6 g/cm) was comparable to that reported previously for hurricane sedimentation along the Louisiana coast (0.8-2.1 g/cm). My findings not only provide insight into how large-scale river floods influence wetland sedimentation, they lay the groundwork for identifying previous flood events in the stratigraphic record.
In Puerto Rico, I applied bulk stable carbon isotope geochemistry of mangrove sediments in the reconstruction Holocene relative sea levels. The modern distribution of δ13C, TOC and C/N values and foraminifera was described from 56 surface samples collected from three sites containing basin and riverine mangrove stands. Vertically-zoned tidal flat (δ13C: -18.6 Â± 2.8 /; TOC: 10.2 Â± 5.7 %; C/N: 12.7 Â± 3.1), mangrove (δ13C: -26.4 Â± 1.0 /; TOC: 33.9 Â± 13.4 %; C/N: 24.3 Â± 6.2), brackish transition (δ13C: -28.8 Â± 0.7 /; TOC: 40.8 Â± 11.7 %; C/N: 21.7 Â± 3.7), and freshwater swamp (δ13C: -28.4 Â± 0.4 /; TOC: 42.8 Â± 4.8 %; C/N: 17.0 Â± 1.1) environments were identified from the modern transects with distinct δ13C, TOC and C/N values. Foraminiferal groups could not be used to interpret the fossil record, although the ratio of foraminifera to thecamoebians (F/T) was used along with δ13C, TOC and C/N values to refine the distinction between brackish and freshwater environments. Using linear discriminant analysis (LDFs), I applied the δ13C TOC, C/N and F/T distributions to reconstruct paleomangrove elevation with confidence in the continuous section of mangrove peat, with a vertical uncertainty of Â± 0.12 m. I demonstrate that δ13C, TOC, and C/N values can be used along with simple microfossil metrics to reconstruct RSL in tropical environments, where records of this kind are limited.
I further explored the use of bulk stable carbon isotope geochemistry for reconstructing Holocene relative sea levels in the Thames Estuary, UK. The modern distribution of bulk stable carbon isotope geochemistry was described from vegetation and surface sediment samples collected from four coastal wetlands of the Thames Estuary and Norfolk, UK. Statistically distinct bulk sediment δ13C, TOC and C/N values were observed among these elevation-dependent environments due to the relative incorporation of in situ vascular vegetation versus allochthonous particulate organic matter and algae, which varied with the degree of tidal inundation and salinity. I compiled a regional database of 349 bulk sediment δ13C, TOC and C/N analyses from published and unpublished data in the UK and the U.S. Atlantic and Pacific coasts and suggest variations among regions may be related to the dominance of C4 vegetation, climate, and local environmental conditions. The range of δ13C, TOC and C/N values from tidal flat/low marsh (δ13C: -24.9 Â± 1.2 /; TOC: 3.6 Â± 1.7 %; C/N: 9.9 Â± 0.8), middle marsh/high marsh (δ13C: -26.2 Â± 1.0 /; TOC: 9.8 Â± 6.7 %; C/N: 12.1 Â± 1.8), brackish transition (δ13C: -27.2 Â± 1.2 /; TOC: 26.1 Â± 11.5 %; C/N: 14.1 Â± 1.9) reed swamp (δ13C: -28.1 Â± 0.3 /; TOC: 39.1 Â± 10.3 %; C/N: 13.8 Â± 1.1) and fen carr (δ13C: -28.8 Â± 0.7 /; TOC: 29.1 Â± 17.1 %; C/N: 15.8 Â± 3.3) were used along with microfossils (foraminifera, pollen, diatoms) to interpret a Holocene sediment core collected from the Thames Estuary.
Khan, Nicole, "Environmental and Sea-Level Reconstruction in Temperate, Subtropical, and Tropical Coastal Wetlands Using Bulk Stable Carbon Isotope Geochemistry and Microfossils" (2013). Publicly Accessible Penn Dissertations. 1329.