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During the past year we have utilized ground-penetrating radar (GPR) transects, sediment cores, and radiocarbon dates to document the stratigraphy and evolution of the Tijucas strandplain in southern Brazil. A set of five deep cores (18 to 22 m), extending through the entire Holocene strandplain and marine deposits and penetrating into the underlying Pleistocene fluvial sediments, shows that more than 75% of the basinal-fill sequence consist of mud discharged by the Tijucas River. Twenty-seven radiocarbon dates indicate shoreline progradation rates accelerated from 0.5 in its early construction to 2 m/yr during the last 1 ka due to decreasing accommodation space. The strandplain is composed of sandy beach-ridge sediments and chenier deposits that range from 4 to 8.0 m in thickness. In GPR sections, strandplain sequences are dominated by low-angle, seaward-dipping clinoforms having an average spacing of approximately 1 m. Although the relationship between subsurface reflector geometry and spacing and shoreline progradation has not been fully studied, one hypothesis is that layers producing the GPR reflectors are formed annually. If this is true, this strandplain may be similar to tree rings in its ability to capture annual growth (sedimentation) patterns. GPR transects and sediment core data suggest that the overall composition (beach-dune ridges versus cheniers) of strandplains is primarily a product of changing sediment regime, whereas variations in slope and spacing of clinoforms are due to variable wave and nearshore processes as well as the infrequent major meteorological and oceanographic events.

Our present work in southern Brazil has produced several important findings: 1. The Pleistocene 5e highstand shoreline stands at + 8 m elevation (above mean sea level) and is identified by a semi-continuous 4-m high dune ridge that exhibits seaward dipping clinoforms in ground-penetrating radar (GPR) sections, 2. Mid-Holocene highstand shoreline is at + 3 m and is separated from the Pleistocene ridge by a former 400 m wide muddy lagoon. The Mid-Holocene highstand was characterized by a transgressive barrier system with extensive washovers that extend 100 m into the muddy lagoon. Seaward prograding beach ridges truncates these units, 3. The thickness of the beachface and shoreface units decreases from 7 to 8 m in the landward, oldest part of the plain to less than 5 m toward the coast, and 4. The sediment composition of the plain gradually changes from dominantly sand in the form of beach ridges in the older very upper plain to sand ridges and cheniers in the mid plain to a very muddy plain with occasional cheniers at the coast. 5. Tijucas strandplain sediments contain ample terrestrially-derived sedimentary organic matter, including abundant fatty acids, sterols and n-alkanes.  The hydrogen isotopic signatures of terrestrially-derived biomarkers are controlled by climate drivers, primarily regional precipitation patterns and temperature. These signatures are being linked in time with other high-resolution climate records obtained from other sites in southern and eastern South America, particularly a speleotherm record from nearby Sao Paulo.

The alternation between sandy beach-ridge construction and chenier formation occurred repeatedly throughout the past 5800 year history of the Tijucas strandplain, representing geomorpholgically significant changes in the evolution of this coast. Our initial data suggest that transformations between sedimentary regimes are abrupt, perhaps taking place over decadal time scales, and signifies dramatic and long-lasting changes in the local hydraulic regime. Given that similar strandplains occur throughout the southern oceans, they likely record the crossing of unique geomorphic and climatic thresholds whose signatures may be obscured or not recorded in less dynamic geomorphic settings.. Thus, our ongoing work is answering the question: How does the geological record inform us about past climate sensitivity and the impact of past abrupt changes in climate under a variety of different boundary conditions?