HIGH RESOLUTION CLIMATE RECORDS OF THE MIO-PLIOCENE BASED ON GEOCHEMICAL SIGNATURES OF TWO STROMATOLITES FROM THE ATACAMA DESERT

Paleoenvironmental information for the eastern Atacama Desert during the Miocene-Pliocene was gathered from two stromatolites that grew within the Opache Formation (5.7 - 2.4 Ma) using geochemical and petrographic information. Carbonate was deposited in repeating wide - narrow band couplets, which likely represent a single year. Metal/Ca ratios, and δ¹⁸O - δ¹³C record fluctuations in seasonal and/or inter-annual rainfall and temperature and are compared to instrumental records from the region. Strontium isotope measurements are used to address changes in the catchment area. Currently daily average temperatures fluctuate less throughout the year than they do diurnally. Precipitation is rare, and water gets to this area as runoff or groundwater flow from the Andes. In the Andes, rainfall occurs in summer months and is strongly influenced by ENSO. Compared to bulk analyses of carbonate from the Opache, each stromatolite plots near the extremity of the δ¹⁸O - δ¹³C array, highlighting the possibility of capturing short-lived changes in water balance. Increases in δ¹⁸O and Mg/Ca ratios often correlate with water salinity and hence aridity, while degassing of CO₂ which also occurs increases δ¹³C. There is a cyclic change in δ¹⁸O and δ¹³C consistent with banding patterns in sample O-52-12 that formed near the top of the Opache as conditions became more arid. The strong correlation between δ¹⁸O and δ¹³C is consistent with seasonal changes in rainfall, but Mg/Ca in O-52-12 exhibits a different pattern which may reflect other processes. The other sample O-25-14, grew in the middle of the Opache under wetter conditions. δ¹⁸O and Mg/Ca both co-vary within the first half of this stromatolite’s life, with larger fluctuations than shown in O-52-12, implying greater changes in rainfall seasonality. However, during the latter part of its lifetime, covariation ceases, which may reflect alteration, or changes in the water’s δ¹⁸O. There is no correlation with δ¹³C.

⁸⁷Sr/⁸⁶Sr represents that of the water, not presence of detrital material. Sample O-52-12, which grew under more arid conditions than sample O-25-14, has lower ⁸⁷Sr/⁸⁶Sr. ⁸⁷Sr/⁸⁶Sr of these samples and Opache carbonate deposited further east converge over time toward value of present-day water, suggesting more water delivery via groundwater as climate became more arid.