RECONSTRUCTING HOLOCENE COASTAL ENVIRONMENTAL CHANGES IN BAJA CALIFORNIA SUR, MEXICO, BASED ON GEOCHEMICAL DATA OBTAINED FROM XRF ANALYSIS: LINKING THE PRESENT AND THE PAST

Understanding the modern distribution and abundance of chemical elements in different coastal environments is vital to the interpretation of XRF stratigraphic datasets for the purpose of reconstructing coastal paleoenvironmental changes. We collected 67 soil surface samples from different environments around two coastal lagoons near La Paz in Baja California Sur, Mexico, and used a handheld XRF scanner to measure the concentrations of 24 chemical elements. The results indicate that soil samples collected from the desert slopes around the lagoons are characterized by high concentrations of lithospheric elements like Ti, Fe, Mn, V, Co, Zr, and Zn, while Sr, Br, Ca, Cl, and S are virtually absent. By contrast, sand samples from the beach are characterized by high concentrations of Sr and Ca, and very low concentrations of the lithospheric elements. Samples from the top of a shell-rich sandy beach ridge contain relatively high concentrations of S, K, Cl, and Zr but low to moderate concentrations of Sr, Ca, Ti, and other lithospheric elements. Samples from mudflats representing dry lake bed sites have geochemical signatures intermediate between those of slope and beach samples, as characterized by moderately high abundance of the lithospheric elements (Ti, Fe) as well as Cl and Br, but typical marine elements like Ca and Sr are rare or absent. Fossil marine shells collected from around the lagoons are marked by high concentrations of Ca (and Cl in one site), low to moderate concentrations of Sr (and Br and S in one site), and the virtual absence of Ti, Fe, Co, and Zr. These modern geochemical results were applied to aid the interpretation of a ~6700-year XRF stratigraphic sequence obtained from Laguna Enfermeria. The whole core is marked by the consistent presence of terrigenous elements like Ti, Fe, Mn, and V, suggesting that soil eroded from the surrounding desert slopes has always been a primary source of sediment supply into this small basin. However, marine elements (Sr, Ca) are absent in the basal peat section and at the top 20 cm of the core, but are abundant in the calcareous sediment in the middle section. The data suggest that the site evolved from a terrestrial wetland to a shallow marine bay during the middle Holocene. The modern backbarrier lake with limited tidal connection to the sea was formed around 1000 years ago.