T306 Ancient Environments of South America

Wednesday, 25 October 2017
8:00 AM - 12:00 PM
Room 604

FOSSIL PHYTOLITH ASSEMBLAGES FROM SOUTHERN PATAGONIA INDICATE CHANGING HABITATS DURING THE MIDDLE MIOCENE CLIMATIC OPTIMUM

Paper number: T306-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

The Middle Miocene Climatic Optimum (MMCO; ca. 17–14.5 Ma) was characterized by high temperature and pCO2, thought to promote warm and wet climates at high latitudes. The Santa Cruz Formation (SCF; 47-52°S) of coastal Patagonia (ca. 17.8–16.6 Ma) represents the southernmost sedimentary record in the world spanning the onset of the MMCO. SCF fossiliferous horizons yield one of the most species rich and well-preserved Cenozoic vertebrate assemblages known. Plant macrofossils, and well preserved phytolith assemblages also occur in the SCF within the same strata as the faunas. These linked fossil records allow us to, for the first time, compare in detail animal and plant paleoecology, vegetation structure, and local climate through the MMCO global warming event in southern South America. Importantly, isotopic data from fossil enamel and bones have been used to estimate Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) and can be directly compared to the vegetation data from phytoliths.

Here, we present preliminary results from analysis of phytolith assemblage composition (% plant functional types) from the main stratigraphic section of coastal SCF, documenting vegetation change during the early part of the MMCO, and interpreting it in light of current climatic reconstructions based on isotopic and faunal data. With the exception of the oldest assemblage, most phytolith assemblages analyzed so far are more or less dominated by forest indicators, including palms. Although there is no clear pattern of variation in overall assemblage composition (e.g., forest indicator: open habitat indicator ratio) through time, different forest indicators dominate different assemblages, which could be explained by vegetation shifts through time, spatial heterogeneity, or both. Grass phytolith assemblages consist mainly of morphotypes from C3 pooid grasses throughout the section, but are characterized by a substantial decrease in morphotype size from older to younger levels, which may reflect decreased water availability. These results are consistent with isotopic data indicating dominantly C3 vegetation and a shift toward a more arid climate characterized by decreased MAP as well as increased MAT during the onset of the MMCO, but suggest that this drying did not result in an expansion of open-habitat grasses.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-298212

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.