Salinity proxy comparison in four tropical biogenic carbonates for suitability in paleohydrologic reconstruction

Abstract:

Climate change is expected to increasingly stress water resources in drought-prone areas such as the American Southwest or Central America. Understanding patterns in precipitation frequency and intensity in the recent past are critical to accurately modeling groundwater recharge potential and managing diminishing resources in conjunction with rapid urbanization (e.g. Yucatan, Mexico). Wet and dry seasons on the Yucatan Peninsula are affected by the Intertropical Convergence Zone (ITCZ) which can exacerbate drought as the climate warms. This study focuses on precipitation patterns during the late Holocene recorded in foraminifera (Ammonia beccarii, Elphidium sp., Quinqueloculina sp.) and ostracods (suspected Hemicyprideis nichuptensis) along a transect in Celestun Lagoon near Merida, Yucatan, Mexico. Submarine groundwater springs feed the north terminus of the lagoon resulting in δ¹⁸O and Mg gradients toward the ocean which oscillate in response to increased or decreased precipitation. Mg/Ca ratios from the north (4.59) and south (3.08) lagoon suggest warmer waters in the shallow northern end, resulting in evaporation and confounding the δ¹⁸O values there (upper lagoon = 2.1‰, middle lagoon = 0.3‰, lower lagoon = 1.5‰). B/Ca ratios remain fairly constant (average range 27.3-33.2) despite increasing B concentrations toward the southern mouth, suggesting a concurrent increase in dissolved calcium down the lagoon and thus a lower Ca signal in spite of the karst source of springs. Of the four species tested, A. beccarii and H. nichuptensis yielded the most similar results ([Ca] ~ 150 ppm). Replicate analyses are needed for Elphidium sp. and Quinqueloculina sp. due to their low sample size and signal variability which suggests they may not be ideal candidates for paleoenvironmental reconstruction in Celestun Lagoon and possible other marginal environments around the Yucatan Peninsula.