T80 Advances in Terrestrial Paleoclimatology and Paleoecology: Geochemical Techniques and Examples Using Inorganic and Organic Molecules in Fossil Soils, Plants, Invertebrates, and Vertebrates II

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 200FG

PALEOSOL CARBON ISOTOPE STRATIGRAPHY AND A ROCK MAGNETIC RECORD OF CLIMATE CHANGE ACROSS THE PALEOCENE-EOCENE BOUNDARY IN THE BOGOTA BASIN, COLOMBIA

Paper number: T80-4
Program: Technical Sessions
Time: 8:50 AM – 9:05 AM

Authors

Abstract

The Paleocene/Eocene Thermal Maximum (PETM) was an event of rapid global climatic change that occurred at 56.3 Ma and is associated with a major negative carbon isotope excursion (CIE) ascribed to massive release of isotopically light carbon into the linked ocean-atmosphere-terrestrial biosphere reservoirs of the global carbon cycle. Terrestrial proxy records from mid to high northern latitudes suggest brief but intense warming across the Northern Hemisphere, but changes in the hydrologic cycle are controversial as paleoprecipitation estimates spanning the PETM vary widely. The Bogota Basin of Colombia preserves a 2 km conformable sequence of paleosols that offers an ideal setting to examine terrestrial climate change across the PETM in the paleotropics (paleolatitude <5˚N). However, existing paleoprecipitation proxies based on paleosols elemental ratios are calibrated only for temperate soils under mean annual precipitation <1500 mm.

We examined the PETM in the Bogota Basin using the carbon isotope composition of bulk paleosol organic matter (n=80) to identify the CIE and the magnetic susceptibility and mineralogy of the paleosols to examine changes in weathering intensity associated with the PETM. Increased precipitation should intensify silicate weathering via hydrolysis reactions involving carbonic acid. For Fe-bearing silicate minerals, these reactions liberate Fe ions, which are often sequestered in Fe-oxides such as goethite and hematite. Increased precipitation could also enhance soil biological activity, increasing production of biogenic magnetite and maghemite. These Fe-oxides all have distinct magnetic properties that are easily measured and potentially can be used as proxies for changes in weathering and precipitation. Carbon isotopes reveal a CIE (~-2‰) in stratigraphic proximity to a tuff dated at 56.13±0.87 Ma. The same interval has a 100% increase in both susceptibility and concentrations of Fe-bearing magnetic minerals. We suggest that a sharp decrease in magnetic mineral concentration immediately after the CIE indicates decreased weathering and precipitation rates after the PETM. Our results suggest that chemical weathering in the tropics, as a proxy for the terrestrial hydrologic cycle, intensified in response to the onset of the PETM and decreased after its termination