Aguirre, Marina

The coastal fringe of Central Patagonia preserves a unique and spectacular succession of landforms discontinuously formed since MIS 11 up to the Holocene. The study area, stretching from 44° 34′ to 44° 54′ S of latitude, is crucial to analyze the complexity of multitemporal shorelines formation and preservation along the Atlantic coast of South America. We used depositional and erosional landforms to get reliable and well chronologically constrained sea level markers. In particular, multistoried swale infillings, produced by a complex relationship between river discharge and marine activity, were considered the most accurate sea level markers. Palaeo sea level elevation was assessed cross checking evidence obtained from different marker types and considering the original position of each of the measured features with respect to its contemporary sea level. A dedicated field measurement of the markers current elevation was necessary, considering the macrotidal regime that currently affects this coastal area. Literature and new data provide an excellent set of dating, useful to chronologically constrain all the palaeo shorelines that have been identified. On the whole five sea level highstands have confidently been referred to Holocene (maximum transgression peak), MIS 5, 7, 9 and 11. Sea level elevation for each of them was stated respectively at 2.5, 7.5, 10.5, 22.5 and 32.5 m a.s.l., but different error bars and levels of accuracy are assigned to each of these estimates, based on innovative criterions that are widely discussed in the text. Our work enabled us to obtain new, self-consistent values of the last 400 ka uplift rates for this coastal tract and to compare them with those calculated by other authors, suggesting for the investigated time span a moderate coastal uplift.

The stratigraphic reconstruction of the northern sector of the Bahía Camarones (Chubut, Argentina) allowed to improve our understanding of the Holocene marine transgression in the area. The first phase of the maximum of the transgression, is interpreted as dominated by the high rate of eustatic rise of sea level until ca. 6-7 ka BP possibly associated to sedimentary starvation as suggested by fossil accumulation. After this first phase, the general trend indicates a progressive fall of the relative sea level after the Middle Holocene high stand as documented in other parts of south America Atlantic coast. Our data, coupled with the robust radiocarbon data set available for the area from literature, indicate three main local steps of coastal aggradation between ca. 6600 and 5400 yr BP (ca. 7000-5600 yr cal BP), ca. 3300 and 2000 yr BP (ca. 3100-1700 yr cal BP), and ca. 1300-500 yr BP (ca. 1000-300 yr cal BP). A significant age gap in coastal aggradation is present between ca. 5300 and 4400 yr BP (ca. 5600-4500 yr cal BP), and perhaps between ca 2000 and 1300 yr BP (ca. 1700-1000 yr cal BP). These can be linked to phases of local sea level fall and/or phases of sedimentary starvation and/or changes in drift transport which can have produced local coastal cannibalization. However, no conclusive data can be advanced. Data obtained from careful measurements of sea level markers represented by the top of marsh and fluvial terraces indicate lower values for the sea level estimation compared with the data set previously proposed for the area. This stigmatizes the fact that field-oriented works are still the priority in the Patagonia coast along with accurate age measurement, especially for obtaining the fundamental information we need for predicting the environmental impact, in these coastal areas, from accelerate sea level rise as effect of global warming.

The Holocene evolution of the Cabo Raso bay (Atlantic Patagonian coast) was reconstructed by means of geomorphological, stratigraphic, and palaeontological analyses, assisted by radiocarbon dating. Six beach ridges were individuated and mapped in the field, as well as some rocky erosional landforms, e.g., inner margins of marine terraces. Thanks to quarry sections, the internal structure of beach ridges, their relationship with continental deposits, and the fossil contents were determined. Two specimens of Aulacomya atra and Brachidontes purpuratus were radiocarbon dated at 6055 and 4500 ± 20 YBP, respectively. The bedrock outcrops at the base of an analysed section allowed us to associate the age of the samples collected to the elevation of the marine transgression surface upon which the entire deposit rests. Because a beach ridge is a regressive form, the elevation of the base of the dated deposit was assumed to be equivalent to or slightly lower than the maximum sea-level stationing, represented by the inner margin of the coheval marine terrace. The altimetric correlation between the base of the beach ridge dated at 6055 ± 20 YBP and the inner margin of the corresponding marine terraces allowed us to constrain the maximum Holocene marine transgression to about 3 to 2 m above sea level. This elevation for the maximum Holocene transgression is lower than that shown by most of the previous data for Patagonian coast, but it shows a crude agreement with recent estimates coming from geophysical models that report, for this area, a departure from the eustatic value of sea level, mainly caused by glacioisostatic process. This means that the employment of marine erosional landforms, associated with other multisource field data, proved to be determinant for reconstructing the sea-level variation in the Patagonian coast.