Date Awarded

2018

Document Type

Thesis

Degree Name

Master of Science (M.Sc.)

Department

Virginia Institute of Marine Science

Advisor

Christopher J Hein

Committee Member

Valier V Galy

Committee Member

Elizabeth A Canuel

Committee Member

James E Perry

Committee Member

Steve A Kuehl

Abstract

Millennial-scale variations in climate forcing are recognized to drive changes in terrestrial processes, and, by extension, impart controls on fluvial sediment loads (e.g., weathering and erosion). However, the impact of decadal- to centennial- scale climate fluctuations on downstream coastal sedimentation patterns and landscape evolution remains unclear. Specifically, the connection between long-term (decades or more) precipitation intensity/seasonality and sediment export from river systems has not been established. This study examines the manner in which sub-millennial-scale fluctuations in precipitation within a river catchment in southern Brazil are recorded in a coastal sedimentary archive. The 5-km wide Tijucas Strandplain formed over the last 5800 years through the rapid reworking of sediment discharged from the Tijucas River in a regime of falling sea level. Within a beach-ridge plain characterized by an overall shift from sand- to mud- dominance (linked to a long-term reduction in wave energy caused by bay shoaling) are nearly 70 distinct transitions between shore-parallel sand- and mud- dominated facies. to assess the potential role of climate forcings (e.g., precipitation patterns) in controlling the delivery of sediment to this coastal system, changes in bulk organic and inorganic characteristics, as well as terrestrial vascular plant wax fatty acid stable hydrogen (δD) and carbon (δ13C) isotopic values, were measured from samples collected across sandy and muddy segments of the plain, and from the modern river, estuary, and bay. Bulk δ13C measurements from modern system samples increase by 3.3 ‰ from the most upstream sampling location to the estuary, indicating considerable mixing and/or replacement of terrestrial with marine organic material prior to sediment preservation in the strandplain. However, C28 fatty-acid δ13C data indicate that the plain faithfully records the terrestrial component of the organic matter pool. Concurrent and equal magnitude shifts in δD values (tracking precipitation source/amount changes) between 2015 and 2017 of both river and beach sediments indicate that river sediments are rapidly transported from the river-estuary interface, onto the modern beach, and preserved within the strandplain. This interannual variability was absent from bulk and biomarker δ13C values, reflecting the slower response of vegetation dynamics to precipitation changes. Modern isotopic data from rainfall characterized by seasonally alternating northeast (distal) and southeast (proximal) sources indicates that relatively isotopically depleted average annual rainfall reflects a higher ratio of austral summer to austral winter precipitation, and thus enhanced seasonality. Long-term changes in these precipitation patterns are observed in strandplain biomarker data: δD values become ~10 ‰ more depleted over the last ca. 2000 years, reflecting a gradual moistening and/or decreased seasonality of regional climate. This is supported by strandplain biomarker δ13C values, which record a long-term shift towards more C3-dominated continental vegetation. Moreover, sand-dominated strandplain segments have biomarker δD values indicative of enhanced seasonality (or aridity), as compared with mud-dominated strandplain segments. It is concluded that drier and/or more seasonal rainfall allows for deeper erosion of soils and enhanced export of sand from the Tijucas River, and that changes in the balance of precipitation source and amounts can force substantial changes in the texture and rate of sediment delivery to the coast.

DOI

http://dx.doi.org/10.25773/v5-xte2-fm94

Rights

© The Author

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