Late-Holocene sea levels from vermetids and barnacles at Ponta do Papagaio, 27° 50′S latitude and a comparison with other sectors of southern Brazil

Abstract

The height difference between paleo-vermetids and present homologous bioconstructions remains as the best Mid- to Late-Holocene paleo-sea level indicator on the Brazilian coast. The vertical range of the vermetid bioconstructions and, consequently, their precision as a sea-level indicator, varies from ±0.1 to ±1.0 m according to local conditions such as wave energy exposure and tidal range. However, live vermetid bioconstructions are scarce or do not occur at several places south of 23°S latitude, which has led to the use of other bioconstructions (e.g. Phragmatopoma caudata) as a homologous reference level, adding uncertainties to the paleo-sea level reconstructions. At the Southeast Brazilian coast, from 25° 30′ S to 28° 37′ S latitude, the precision varies from ±0.5 to ±1.0 m. At the Ponta do Papagaio headland, at 27°50′38″- 44″ S latitude, the occurrence of the subfossil barnacle Tetraclita stalactifera included in the remains of Holocene vermetids (Petaloconchus varians (d'Orbigny, 1839)) bioconstructions allowed the authors to adopt the living fringe of this barnacle species as a homologous reference level and thereby improve the accuracy of paleo-sea level reconstructions (error margin of only ± 0.5 to ± 0.3 m). The paleo-sea level obtained at Ponta do Papagaio by this method varied between +2.4 and 0.8 m. The resultant relative sea-level curve is included in the empiric sea-level envelop previously determined for the Brazilian coast, south of 28°S latitude, and matches with the far-field eustatic sea-level models. It is also similar to other curves defined for Southern Brazil regions such as Laguna-Imbituba (Santa Marta Cape surroundings, 85-40 km to South from Ponta do Papagaio), São Francisco do Sul (180 km North) and Paraná (220–260 km North). However, the height of 2.4 ± 0.3 m at 4 ky BP at Ponta do Papagaio is similar to the two cited regions to the North, but higher than the one at Laguna-Imbituba for the same period. This difference may not be significant, as it is within the ±1.0 m error margin adopted in the inference of the paleo-sea level at Laguna-Imbituba. If it is significant, it can be attributed to a local lowering of relative sea-level by thermosteric and dynamic effects of the Santa Marta Cape coastal upwelling strengthening at the epoch of maximum sea-level. This work also suggests further quantification of the living vertical distribution of vermetids and other species used as reference species (e.g. Tetraclita stalactifera, Phragmatopoma caudata), under different bedrock morphology, wave exposure and oceanographic conditions, as a necessary way to improve the precision of paleo-sea level reconstructions.