Igor Gutiérrez-Zugasti

Determination of sea surface temperatures using oxygen isotope ratios from Phorcus lineatus (Da Costa, 1778) in northern Spain: Implications for paleoclimate and archaeological studies

Changes in oxygen isotope ratios from shell carbonates are mainly dependent on sea surface temperature, which enables the estimation of temperatures during periods of shell growth and helps to determine the season of the year when the mollusk died. The marine topshell Phorcus lineatus (Da Costa, 1778) is commonly found in Holocene archaeological deposits of Atlantic Europe and is one of the most abundant subsistence resources utilized during the Mesolithic in northern Spain. Before applying isotopic techniques to ancient samples, calibration of the past isotopic data and its variability must be performed through the study of modern specimens to test their potential as paleoclimate proxy and their suitability for determining the collection season. Although previous studies performed in the region highlighted the existing relationship between sea surface temperatures and isotopic signatures, no systematic works have been done so far. In this paper, calibration of modern P. lineatus shells from northern Spain was carried out using δ18O analysis. The results showed (1) the existence of a robust inverse correlation between instrumental temperatures (Tmeas) and δ18Oshell (R2 > 0.9), accompanied by the lack of significant dependence from δ18Owater variations (R2 = 0.06); (2) the existence of conditions of (or close to) isotopic equilibrium during the formation of the aragonite in the P. lineatus shells; and (3) that using mean annual δ18Owater values, past temperatures could be calculated with a maximum uncertainty of ±3°C. Moreover, results suggested that P. lineatus generally grew without substantial slow/cessation throughout the year, reflecting the four annual seasons. Therefore, our study not only confirms the potential of oxygen isotope analysis on P. lineatus for paleoclimate reconstruction and archaeological studies highlighted in previous studies but also shows for the first time that the aragonite of those shells grew under conditions of isotopic equilibrium, opening new avenues for future research.

Mg/Ca ratios measured by laser induced breakdown spectroscopy (LIBS): a new approach to decipher environmental conditions

The potential application of Mg/Ca ratios in top shells of the mollusc species Phorcus lineatus (Da Costa, 1778) obtained by Laser Induced Breakdown Spectroscopy (LIBS) has been evaluated as an environmental proxy to reconstruct paleotemperatures and season of capture of molluscs for the first time. All samples were collected from the Cantabrian Sea (Spain). The results were compared with instrumental sea surface temperatures (SSTs) and with a known reliable proxy as the oxygen isotope ratio (δ18Oshell) which is mainly dependent on the SST, obtained from the same shells. Measurements were taken in two different biominerals of the shell (aragonite and calcite) resulting in a correlation between Mg/Ca ratios and SSTs of R2 = 0.43 and 0.44, respectively. Mg/Ca ratios were also studied through a long sequence on three shells collected in autumn 2012. The results show variations in Mg/Ca ratios related to seasonal changes in the SST throughout the year and a good correlation between Mg/Ca ratios and δ18Oshell in two shells (R2 = 0.70 and 0.65, respectively).

Marine Radiocarbon Reservoir Effect in Late Pleistocene and Early Holocene Coastal Waters off Northern Iberia

Radiocarbon dating of closely associated marine mollusk shells and terrestrial material (mammal bones or charred wood) collected from archaeological contexts in northern Atlantic Iberian coastal areas is used to quantify the marine 14C reservoir effect (ΔR) for the coastal waters off the Cantabrian coast (northern Iberia). For the first time, ΔR values were reliably determined for these coastal waters, and also for the first time a ΔR was calculated for the Late Pleistocene in Atlantic Iberia. Pairs of coeval samples of different carbon reservoirs selected from Upper Paleolithic (Late Pleistocene) and Mesolithic (Early Holocene) contexts yielded ΔR weighted mean values of −117±70 14C yr and −105±21 14C yr, respectively. These values show oceanographic conditions characterized by a reduced offset between atmospheric and surface water 14C contents, suggesting a nonexistent or a very weak upwelling and some stratification of the water column. Similar oceanographic conditions have been recorded in other areas of Atlantic Iberia during the Holocene, such as off Andalusian and northwestern Galician coasts. Results not only provide useful information on environmental conditions but also a framework to obtain more precise and reliable absolute chronologies for the Late Pleistocene and Early Holocene in northern Iberia.

Automated measurement of magnesium/calcium ratios in gastropod shells using laser-induced breakdown spectroscopy for paleoclimatic applications

The chemical composition of mollusk shells offers information about environmental conditions present during the lifespan of the organism. Shells found in geological deposits and in many archeological sites can help to reconstruct past climatic conditions. For example, a correlation has been found between seawater temperature and the amount of some substituent elements (e.g., magnesium, strontium) in the biogenerated calcium carbonate matrix of the shell, although it is very species-specific. Here we propose the use laser-induced breakdown spectroscopy (LIBS) to estimate Mg/Ca ratios in modern specimens of the common limpet Patella vulgata. An automated setup was used to obtain a sequence of Mg/Ca ratios across a sampling path that could be compared with the seawater temperatures recorded during the organism’s lifespan. Results using four shells collected in different months of the year showed a direct relationship between the Mg/Ca ratios and the seawater temperature, although the sequences also revealed small-scale (short-term) variability and an irregular growth rate. Nevertheless, it was possible to infer the season of capture and the minimum and maximum seawater temperatures from the LIBS sequences. This fact, along with the reduction in sampling and measurement time compared with other spectrometric techniques (such as inductively coupled plasma mass spectrometry [ICP-MS]), makes LIBS useful in paleoclimatic studies.

Mesolithic dwelling structures from methodological approaches to archaeological interpretation

The study of Mesolithic settlements is a key aspect to understanding the articulation of social and economic organizations of the last hunter-gatherers. Analyzing the spatial organization of settlements increases indeed our knowledge of activity areas, subsistence strategies, seasonality of occupations and social organization. At another “reading” level, spatial organization is a direct reflection of a “Weltanschauung”. It necessarily reproduces the links between humans and non-humans, both in the material world and elsewhere. Describing a Mesolithic habitat means gaining access to a form of world organization: obviously, the process is anything but simple!