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During the past year we have expanded significantly upon our previous work at Tijucas and begun study of the nearby comparison site of Pinheira, Santa Catarina, Brazil. In previous years at Tijucas, we have determined that, on a broad scale, the sediment composition of the strandplain 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. At finer resolution, we found that 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. Field investigations conducted over the past year have sought to enhance our understanding of nearshore processes affecting modern chenier development at Tijucas. A series of 42 surface sediment samples and 3 shallow cores were collected along 4 shore-normal transects in the shorface at Tijucas. Of these, greater than 90% were composed of mud or liquid mud; sandy sediments were found only in the nearshore regions most proximal to the Tijucas River mouth. The deposition of thin (10 – 30 cm) lenses of fine to medium, shell-rich sand along the river-proximal shoreline after several high energy events over this past year provided further evidence of the occasional onshore reworking of sand. These observations confirm the modern dominance of muddy sediment at Tijucas; explain the presence of numerous thin sand layers within the 20 m thick mud sequence found in deep cores in previous years; and provide the final data needed for our understanding of chenier and strandplain development at Tijucas. Such background is crucial as we engage in our final stage of work at Tijucas: addressing how the geological record informs us about past climate sensitivity and the impact of past abrupt changes in climate under a variety of different boundary conditions.

At Pinheira, an eastward-facing strandplain located 60 km south of Tijucas and in an embayment protected to the north by Ilha de Santa Catarina and fronted by several bedrock headlands, geophysical and sedimentological studies have uncovered the existence of a paleo-barrier island system that has illuminated the nature of sea-level fall in the Southern Hemisphere during the mid- to late-Holocene. Here, gross strandplain morphology was determined based on Geographic Information Systems (GIS) and ground-based geomorphic surveys supplemented with approximately 20 km of global positioning system with real-time kinematics (RTK-GPS) data. Overall strandplain stratigraphy was determined from four deep (16 m) cores collected using a combination wash boring / direct push system. Samples were collected for sediment and radiocarbon analysis at sub-meter intervals in the direct-push core sections. Additionally, more than 11 km of ground-penetrating radar (GPR) data were collected and topographically corrected using RTK-GPS data. Based on these, target locations were chosen for detailed study. Additional GPR surveys were ground-truthed with 13 vibracores (each 3 – 6 m deep) and a series of 1 – 6 m deep auger cores.

Results from this work at Pinheira reveal that, unlike the riverine-dominated system at Tijucas, the evolution of this strandplain was marked by continuous sediment supply from nearshore under conditions of generally falling sea-level. Unlike at Tijucas, neither Marine Isotope Stage 5e nor Holocene highstand barriers are evident at Pinheira; rather, sediment was deposited directly at the bedrock-controlled shoreline at the Holocene highstand. The following forced regression resulted in the deposition of a regressive sequence containing around 0.5 km³ of sediment that formed from the deposition of continental shelf sediments eroded and reworked onshore as wave base lowered over the past 5000 years. The ultimate sediment source for the development of this strandplain is the Rio de la Plata, located at the border between Uruguay and Argentina, over 1000 km south of Pinheira. Located in a highly protected embayment and along a coastline impacted by few, infrequent storm events, progradation of the strandplain occurred via the shallow water deposition of shoreface sands followed by the welding of landward-migrating bars that formed the beach and foreshore. Foredune ridges built upon these beach ridges. Ridge morphology and spacing are consistent across the plain and represents a relatively stable, continuous progradation rate of approximately 0.8 – 1.0 m / year. Disturbances to the gradual formation of this strandplain are due to small oscillations in the rate or direction of sea level change or to the crossing of wave regime thresholds that result in the rapid mobilization and onshore reworking of larger quantities of nearshore sand than otherwise occurred during the gradual regression. One such perturbation is imprinted on the strandplain in the form of a barrier-lagoon-inlet system that developed 3.5 – 3.6 ka cal yr BP. GPR and sediment cores revealed the existence of a lagoon, tidal inlet, a 100-m wide barrier ridge, and numerous overwash units within the strandplain sequence. This barrier island system was active for at most 700 years when the inlet was rendered inactive and the barrier and lagoon were stranded inland by a return to falling sea levels and shoreline progradation. The results of this work are currently in the final stages of preparation, to be submitted to an academic journal in 2010.