Increased Precipitation over the Yucatan Peninsula Inferred from the Spatial and Temporal Distribution of Late Holocene Foram Assemblages

Abstract:

As global climate warms, IPCC predictions suggest dry and seasonally dry regions will become dryer, increasing stress on water resources by growing urban populations (e.g. Southern California; Yucatan, Mexico). This study aims to reconstruct paleohydrologic trends during the late Holocene using foraminifera assemblages in Yucatan, Mexico to determine drought susceptibility in a region affected by migration of the Intertropical Convergence Zone (ITCZ). The ITCZ affects precipitation over the Yucatan Peninsula, potentially decreasing groundwater infiltration and thus reducing discharge in submarine springs at the peninsula margins. The field site Celestun Lagoon near Merida, Yucatan, Mexico, is dominated by spring and groundwater inputs at the northern terminus and opens to the Gulf of Mexico at the southern end resulting in a strong salinity gradient dependent on freshwater influx. We analyzed the foram assemblage in the top 6 cm of a set of 7 cores collected along a lagoonal transect (from the mouth to the head) and plotted the relative abundances of Ammonia beccarii, Elphidium sp., Quinqueloculina sp., and the ostracod Hemicyprideis cf. nichuptensis against site location. A. beccarii abundance increases from 40% near the head to 70% near the middle of the lagoon before dropping to <10% toward the saline lagoon mouth (high abundance in brackish salinity). Quinqueloculina sp. increases from 0% at the upper lagoon to nearly 40% near the mouth showing an opposite trend along the same transect. Elphidium sp. showed no clear trend (abundance range 10-28% throughout), and lowest H. nichuptensis abundance occurred at the middle of the lagoon (10%) increasing to the north and south (up to 60%). The inversely correlated spatial distribution between A. beccarii and Quinqueloculina sp. occurs vertically in an upper lagoon long core from 125-96 cm below sediment-water interface (¹⁴C age dates 3968-2820 ka), implying a decreasing salinity over ~1000 yrs and increased precipitation over the peninsula feeding the springs. The relationship between decreasing Elphidium sp. abundance and salinity over the same time interval is unclear; ostracod trends are being measured. This assemblage data corresponds to low δ¹⁸O values in Yucatan lakes suggesting low evaporation and increased wet conditions during the same time period.