Holocene sea-surface temperatures and related coastal upwelling regime recorded by vermetid assemblages, southeastern Brazil (Arraial do Cabo, RJ)

In places where ocean currents cause upwelling, nearshore sea-surface temperatures (SST) are often cooler than nearby offshore waters. In the Arraial do Cabo Bay coast upper Holocene aragonitic vermetids are represented by monospecific clusters of overgrowing Petaloconchus varians occurring in supratidal/intertidal carbonate and mixed siliciclastic-carbonate deposits. Based on stable isotope composition (δ¹³C, δ¹⁸O) of fossil vermetid shells, radiocarbon ages and altimetric survey, the upper Holocene upwelling system of the Cabo Frio area (southeastern Brazil, Rio de Janeiro) is assessed. In the studied region, at 3700 cal. years BP the maximum relative sea level (RSL) was +4.0 m with a SST of 20.7 °C. Subsequently, vermetid-based SST decreased from ~22.8 to ~17 °C (at ~3300 cal. years BP), with the coldest temperatures recording a strong upwelling event at around 2000 cal. years BP when the RSL was at +2 m. The intensification in upwelling water masses is identified by the ¹³C enrichment, along with higher δ¹⁸O, in the vermetid shells. The decreasing SST trend assessed from ~3300 to ~2000 cal. years BP can be related to more frequent South Atlantic Coastal Water intrusions on the surface layer in the mid-shelf, increasing the nutrient concentration in the upper layer. From ~1900 to ~1300 cal. years BP, a higher SST up to ~21 °C occurred during the continuous sea-level fall.