AGU Abstracts

2014

Dynamical changes in the tropical Pacific are hypothesized to have exerted an important influence on climate of the past millennium. However, direct proxy evidence in support of this hypothesis from the Eastern Tropical Pacific remains sparse. Here we present a unique 1000+ year continuous record of oceanic mixed layer temperature and its variability from a sediment multi-core collected in 2009 near the Galápagos Islands. The study location sits in the center of action of the El Niño Southern Oscillation (ENSO), a major driver of tropical and global climate variability. We analyzed Mg/Ca ratios of multiple specimens of the mixed-layer dwelling foraminifera Globigerinoides ruber to reconstruct mean sea surface temperature (SST) over the past 1000 years. We also analyzed δ18O from individual specimens of G. ruber from the same samples to assess mixed layer temperature variability during the same period. Both the multi-shell Mg/Ca and single-shell δ18O reveal statistically significant and systematic changes during the past millennium. The Medieval Climate Anomaly (MCA, ~1100-1400 CE) features a sustained period of ~1°C cooler mean surface temperatures and reduced variability (by up to 35%) compared to the late 20th century. Little Ice Age (LIA) mean mixed layer temperatures were comparatively warmer and much more variable than the MCA. Intervals with greater variability than modern, as well as intervals with lower than modern variability are both present in the LIA. The most recent sediment interval corresponding to the period 1985-2009 CE has the highest mean SST of the past 1000 years, although it is within error of temperatures from ~1000 CE. An estimate of the zonal SST gradient of the tropical Pacific based on this Mg/Ca data and similar data from the western Pacific supports a pattern of enhanced zonal gradient during the MCA and reduced gradient during the LIA. We explore the implications of these results for tropical Pacific dynamics in the context of external forcing from volcanic and solar variability.

AGU Fall Meeting 2014 Oral Presentation, Multiproxy Records for Climatic and Oceanic Reconstructions I, 12/18/2014 9:30AM.

2013

The dynamic response of the El Niño Southern Oscillation (ENSO) to varying solar and volcanic forcing is thought to be an important influence on climate during the Medieval Climate Anomaly (MCA), but proxy evidence of ENSO variability during the MCA is sparse. Insight into past oceanographic variability can be provided through the analysis of δ18O from individual foraminifera found in deep-sea sediments. This approach is applied here to a high-resolution (>10cm/ky) multi-core from the Eastern Tropical Pacific (MC42) near the Galapagos Islands (01° 15.58’S, 89° 41.13’W, 615m depth). At this location, sea surface variability is strongly influenced by ENSO and the seasonal cycle. δ18O from individual mixed-layer dwelling Globigerinoides ruber was analyzed at multiple time intervals throughout the instrumental era and the MCA (900-1350CE). δ18O from instrumental-era samples captured the variability and full range of oceanographic conditions predicted by δ18O calculated from sea surface temperature and salinity reanalysis data, including peak El Niño and La Niña conditions. δ18O from individual foraminifera from MCA samples display variability reductions from 27% to 33% compared to late twentieth century values, as well as reduced range and fewer δ18O outliers, suggesting weaker ENSO activity. This reduction in variability is accompanied by an increase in mean δ18O and is consistent with existing paleoclimate reconstructions and the modeled response of the tropical Pacific to increased solar forcing during the MCA. Additional data other pre-instrumental time intervals will be presented to characterize ENSO dynamics during the past millennium.

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