Mixed siliciclastic-carbonate systems are globally distributed throughout the tropics, but have received limited research attention on active sedimentation. A detailed examination of sedimentation near La Parguera, Puerto Rico has been conducted to better understand this mixed system on the border of the Caribbean Sea, along the southwest coast of the island. This study includes an assessment of the sediment composition and texture of material accumulating on the La Parguera seabed, as well as an examination of excess (210)Pb-activity profiles and accumulation rates. Variations in grain-size distribution and carbonate and loss-on-ignition (LOI) percentages are examined on the shelf. More specifically, an increase in carbonate percentage and a decrease in the LOI fraction in the seaward direction are evident. Excess (210)Pb-activity profiles display steady-state and non-steady-state profiles. Steady-state profiles are most common and display a range of thicknesses in surface-mixed layers (3 - >20 cm). Non-steady-state profiles are observed in some nearshore settings, reflecting episodic deposition or human influence. Greatest sediment accumulation rates are generally found close to shore, where maximum rates approach 0.5 cm y(-1). Sediment accumulation rates in more seaward reef areas are approximately 0.2 cm y(-1). Mass accumulation rates calculated from the composition and accumulation rate data indicate there is a seaward decrease in terrestrial (non-carbonate and LOI-free) sediment flux to the seabed. Fluxes of terrestrial sediment in nearshore areas are typically several times higher (>0.05 vs. 0.01 g cm(-2) y(-1)). These trends in composition and mass accumulation reflect sediment supply and dispersal from terrestrial and marine sources along with the reduced wave climate from reef sheltering. A preliminary sediment budget suggests the majority (61%) of terrestrial sediment supplied to the shelf is stored locally. Sediment accumulating on the shelf is principally carbonate (85%) and is assumed to be marine-derived. Terrestrial (12%) and LOI (3%) material represent considerably smaller, but significant constituents of the sediment stored. Collectively, data suggest terrestrial sedimentation is a lesser, but increasing sedimentary component of the La Parguera mixed siliciclastic-carbonate setting.
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