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US Mid Atlantic Coastal Plain Multi-Proxy Sclerochronological Evidence for Plio-Pleistocene Regional Warmth: US Mid Atlantic Coastal Plain Ian Z. Winkelstern A thesis submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geological Sciences Chapel Hill 2012 Approved by: Dr. Donna M. Surge Dr. Joseph G. Carter Dr. Justin B. Ries ©2012 Ian Z. Winkelstern ALL RIGHTS RESERVED ii ABSTRACT IAN Z. WINKELSTERN: Multi-Proxy Sclerochronological Evidence for Plio-Pleistocene Regional Warmth: US Mid Atlantic Coastal Plain (Under the direction of Donna M. Surge) Assessments of global Pliocene climate using conventional proxies indicate prevalent warmth at mid to high latitudes. However, it remains poorly understood how these climate changes were manifested on a regional basis and on annual timescales. Oxygen isotope ratios (δ18O) of fossil Mercenaria spp. (bivalve) shells enable reconstruction of growth temperature to estimate sea surface temperature (SST) at seasonal resolution, and comparing growth characteristics of fossil and modern populations can provide independent SST estimates. Fossil shells selected from the Yorktown Formation (Pliocene, early Piacenzian, ~3.3-3.0 Ma, warm interval) and the Chowan River Formation (Pleistocene, Gelasian, 2.4-1.8 Ma, subsequent cooling) of the US Mid Atlantic Coastal Plain document warmth and reduced seasonality relative to modern. Growth analyses of fossil populations likely exclude the possibility of more extreme (>25°C) average annual conditions. These results support model results of increased meridional heat transport during the Pliocene. iii ACKNOWLEDGEMENTS I am very grateful for the guidance of Dr. Donna Surge, whose willingness to share time, knowledge, and ideas made this work possible. I am also indebted to Joel Hudley for crucial help in developing the project through all its phases. In addition, thanks go to Dr. Jose Rafael Garcia-March and Dr. Ting Wang for assisting with field collection and growth analysis. Advice and reviews from Dr. Joe Carter and Dr. Justin Reis further improved this thesis. I thank Lora Wingate at the Stable Isotope Laboratory, University of Michigan, for conducting the isotopic analyses. Partial funding was provided by National Science Foundation grant #AGS-1103371 awarded to Dr. Surge and by student research grants awarded to me by the UNC Geological Sciences Martin Fund, The Geological Society of America, and The Paleontological Society. Finally, I thank my Mom and Dad for their endless encouragement. iv TABLE OF CONTENTS LIST OF TABLES…………………………………………………………………………...vii LIST OF FIGURES…………………………………………………………………………viii LIST OF ABBREVIATIONS AND SYMBOLS…………………………………………… ix Chapter I. MULTI-PROXY SCLEROCHRONOLOGICAL EVIDENCE FOR PLIO-PLEISTOCENE REGIONAL WARMTH: US MID ATLANTIC COASTAL PLAIN....................................1 Abstract..............................................................................................................1 Introduction........................................................................................................2 Geologic Context...............................................................................................5 Ecology of Mercenaria......................................................................................7 Methods.............................................................................................................8 Fossil Growth Rate Analysis...................................................................9 Microsampling and Isotopic Analysis..................................................12 Temperature Calculations from Oxygen Isotope Ratios.....................................................................................................13 Results..............................................................................................................16 Growth Comparison.............................................................................16 Isotopic Ratios of Hinge Plate and Shell Margin.................................16 v Isotopic Ratios of Yorktown and Chowan River Shells....................................................................................................17 Discussion........................................................................................................18 Growth Curves and Sea Surface Temperature.....................................18 Comparison between Hinge Plate and Shell Margin...........................20 Oxygen Isotope Ratios and Seasonal Variability.................................21 Multi-proxy Assessment of MACP Plio-Pleistocene Paleoclimate.............................................................24 Conclusions......................................................................................................26 APPENDIX: Oxygen and Carbon Isotope Ratios of fossil Mercenaria Shells........................................................................................................................................36 REFERENCES........................................................................................................................54 vi LIST OF TABLES Table 1. Modern Temperature and Growth Data...................................................................35 2. Calculated Temperatures Based on Isotopic Data...................................................36 3. Calculated fossil von Bertalanffy growth parameters..............................................37 vii LIST OF FIGURES Figure 1. Stratigraphy and Paleogeography............................................................................27 2. SST-Latitude Relationship and k value – SST Relationship...................................28 3. Fossil von Bertalanffy Growth Curves....................................................................29 4. Isotopic Profiles.......................................................................................................30 5. Shell Margin and Umbo Isotopic Comparison........................................................31 6. Multi-Study Results Comparison.............................................................................32 viii LIST OF ABBREVIATIONS AND SYMBOLS ‰ per mil or parts per thousand ± plus or minus oC degrees Celsius 13C carbon isotope ratio 18O oxygen isotope ratio 18 Oarag oxygen isotope ratio of aragonite 18 Ow oxygen isotope ratio of water et al. and others i.e., that is kyr kiloyear Lmax maximum length of organism gmicrogram M. camphechiensis Mercenaria campechiensis M. Mercenaria Mercenaria mercenaria Ma megaannum MACP Mid Atlantic Coastal Plain MPWP Mid Pliocene Warm Period oN degrees North latitude NBS National Bureau of Standard psu practical salinity units SST sea surface temperature ix t0 initial organism length VPDB Vienna Pee Dee Belemnite VSMOW Vienna Standard Mean Ocean Water x CHAPTER 1: MULTI-PROXY SCLEROCHRONOLOGICAL EVIDENCE FOR PLIO- PLEISTOCENE REGIONAL WARMTH: US MID ATLANTIC COASTAL PLAIN ABSTRACT Assessments of global Pliocene climate using conventional proxies indicate prevalent warmth at mid to high latitudes. However, it remains poorly understood how these climate changes were manifested on a regional basis and on annual timescales. Oxygen isotope ratios (δ18O) of fossil Mercenaria spp. (bivalve) shells enable reconstruction of growth temperature to estimate sea surface temperature (SST) at seasonal resolution, and comparing growth characteristics of fossil and modern populations potentially provide independent SST constraints. Fossil shells were selected from the Rushmere Member of the Yorktown Formation (Pliocene, early Piacenzian, ~3.3-3.0 Ma, warm interval) and the Chowan River Formation (Pleistocene, Gelasian, 2.4-1.8 Ma, subsequent cooling) of the US Mid Atlantic Coastal Plain (MACP). The coldest winter months recorded in the Yorktown shells averaged 17±2°C and the warmest summer months averaged 25±2°C. Chowan River shells recorded the coldest winter months averaging 12±2°C and the warmest summer months averaging 21±2°C. Modern winter and summer SST is 6±2°C and 24±2°C, respectively. The vast 1 majority of data do not approach temperature growth limits of Mercenaria; thus, Yorktown and Chowan River shells document reduced seasonality relative to modern conditions. Population growth analyses do not provide reasonable quantitative SST estimates. However, they show that fossil populations did not grow like those living in very warm environments today, likely excluding the possibility of more extreme (>25°C) average annual conditions. Isotopic results support model results of increased meridional heat transport during the Pliocene. INTRODUCTION The Mid Pliocene Warm Period (MPWP, ~3.3 to 3.0 Ma) is among the most critical geologic intervals for studying the Earth during a warm, relatively high atmospheric CO2 state (Salzmann et al., 2011; Jansen et al., 2007). Paleoclimate data available from the United States Geological Survey Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) project and a variety of independent studies enable modeling of global climatic conditions during the Pliocene (Dowsett et al., 2011). Lacking in these data, however, are sufficient high-resolution time series that can potentially provide more detailed information about annual temperature cyclicity (seasonality) and regional climate variability. Along the Mid Atlantic Coastal Plain (MACP) of eastern North America, qualitative and quantitative paleoclimate reconstructions
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