Winter-Dominated Little Ice Age Cooling in the Northern Gulf of Mexico Determined from Mg/Ca in Planktonic Foraminifera

Rita Marie Crouch

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, Department of Environmental Science and Policy, College of Arts and Sciences, University of South Florida St. Petersburg, March 31, 2014.


Reconstructing late Holocene sea-surface temperature (SST) establishes a baseline for preindustrial climate change, which has important applications for climate models and forecasts to predict current anthropogenic influences. The Medieval Warm Period (MWP) (~900 to 1300 AD) and Little Ice Age (LIA) (~1400 to 1800 AD) are the two most recent preindustrial climate change extremes. Mean-annual SST changes in the northern Gulf of Mexico (GOM) during this time period are well known, but the seasonal distribution of these changes are not. This study presents the first paired record showing seasonal distribution of temperature changes over the past millennium in the northern GOM. Mg/Ca derived from planktonic foraminifera species Globigerinoides ruber (white and pink varieties) and Globorotalia truncatulinoides (non-encrusted) yield a record of mean-seasonal (winter and summer) and mean-annual sea-surface temperature (SST) from the northern GOM. During the MWP, all three records of mean-annual and mean-seasonal (winter and summer) SST were within error of modern observed SSTs, with a difference between summer and winter SSTs of ~3ºC. During the LIA, the difference between summer and winter SSTs was large (~8ºC), with mean-winter and mean-annual SST consistently colder than modern observed SSTs. The data presented clearly show muted summer SST changes coupled with enhanced winter and mean-annual SST changes during the LIA, indicating that winter seasonality drove the observed mean-annual record and dominated the observed LIA cooling in the northern GOM. These results also indicate that enhanced winter seasonality plays an important role in the climate of the GOM region.