Modern influences on simulated Andean rainfall δ18O and application to a Holocene speleothem record
Abstract
The central Peruvian Andes is a region where there are divergent interpretations of the dominant influence on stable oxygen isotopes of precipitation (δ18Op) for modern and geologic timescales. Due to the wealth of paleoclimate proxy records in the region but the lack of observational measurements for δ18Op, we utilize the isotope enabled ECHAM4 simulation from the SWING1 experiment to investigate seasonal to multidecadal influences on δ18Op from 1870-2003. Modeled precipitation and temperature results are validated against available station data in the region. We compare simulated δ18Op at the study site to temperature, precipitation amount, and water vapor δ18O locally and for tropical South America in general. Local precipitation amount and temperature effects are shown to be rather weak over interannual timescales. Instead, more substantial influences are linked to the degree of moisture recycling and rainout upstream in the Amazon Basin, both of which are associated with the intensity of the South American Summer Monsoon (SASM). The significant role of the El Niño-Southern Oscillation (ENSO) on Altiplano climate is further supported for interannual timescales. However, for multidecadal timescales, the Altiplano δ18Op - ENSO relationship exhibits non-stationary characteristics and Atlantic sea surface temperature gradients exert a more sustained influence when the amplitude of ENSO variance is low. Important implications for paleoclimate records are that Altiplano δ18Op is an indicator of regional climate and SASM intensity, which are ultimately linked to the latitudinal position of the Intertropical Convergence Zone (ITCZ). Speleothem samples were recently collected in the Peruvian Altiplano (12°S, 76°W, ~3700m elevation) and have chronologies determined by U-Th dating that span distinct intervals of time over the last ~250,000 years. A high-resolution δ18O profile is based off of 1,400 samples from two speleothems covering 0-7 kya. Over orbital timescales, oxygen isotopes are enriched (-12%) during the mid-Holocene and depleted (-14%) during the late Holocene. This trend follows that of local insolation and demonstrates that an increase in insolation intensifies the SASM. Our reconstruction also reveals prominent centennial scale deviations from orbital variance, including rapid enrichments at 2 kya and into the modern era. These isotopic fluctuations are in the opposite sense from that expected for ENSO forcing when compared to sea surface temperature warming in the western equatorial Pacific. Instead, we invoke north-south migration of the ITCZ over the Pacific and Atlantic Oceans to explain such variations. This research sheds new light on the influences of δ18Op and SASM intensity over modern and geologic timescales.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPP24B..03K
- Keywords:
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- 1620 GLOBAL CHANGE / Climate dynamics;
- 9360 GEOGRAPHIC LOCATION / South America