Fig 1.
Location of Guerrero, Laguna Mitla, and Acapulco (a.) along Mexico’s southern Pacific coast (inset). A satellite image (USGS National Map Viewer) of Laguna Mitla is shown (b). A ~60 cm thick, predominantly sand unit attributed to a marine event was discovered in a sediment core (ACA04-06) extracted 5 km from the coast at the location marked with a red star [12]. Sedimentary evidence of a marine intrusion was also reported from a core (ACA03-02) extracted 4 km from the coast at the location marked with a red square [14]. The six coring locations discussed in this paper are marked by white dots. Exposed relict beach ridge remnants Isla Magueyes (IM) and Isla El Conejo (IEC) are labeled.
Table 1.
Radiocarbon dating results for cores 2–6.
Calib 7.0 [24] and Intcal 13 [25] were used for calibration.
Fig 2.
Elemental concentrations are paired with LOI results for all cores. Note scales vary between cores.
Fig 3.
Elemental concentrations of surface samples.
Surface samples were collected from the coastal beach and adjacent of the coring sites. Concentrations from surface samples belonging to the same coring site were averaged and plotted. Titanium concentrations decrease eastward across the transects. Concentrations of sulfur, strontium and calcium, commonly-used marine indicators, are higher in the landward sites (2, 4, 6) than in the paired seaward sites (1, 3, 5). Beach samples contain relatively low concentrations of sulfur, strontium and calcium.
Fig 4.
Elemental concentrations of sediment cores.
Titanium, sulfur, calcium, and strontium concentrations vs age for seaward core 5 (black), and landward cores 4 (green) and 6 (red). Titanium concentrations (a.) are higher in core 4, nearest the input stream, than cores 5 and 6, farther east. Concentrations of sulfur (b.), calcium (c.), and strontium (d.) are generally higher in landward cores 4 and 6, than the seaward core 5. Spikes in marine elements are largely synonymous with shells (whole, hash) and/or sand, all largely absent in core 5 with the exception of basal sediments. A major marine event was posited from two landward sites, marked by a star and square in Fig 1 [12, 14]. A matching event is absent in the cores from this study, with no sand occurring at any site from 3000–4000 years BP.