MARSH SEDIMENT ACCUMULATION AND ACCRETION ON A RAPIDLY RETREATING ESTUARINE COAST by Conor McDowell A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Master of Science in Marine Studies Winter 2017 © 2017 Conor McDowell All Rights Reserved ProQuest Number:10256075 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. ProQuest 10256075 Published by ProQuest LLC ( 2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, MI 48106 - 1346 MARSH SEDIMENT ACCUMULATION AND ACCRETION ON A RAPIDLY RETREATING ESTUARINE COAST by Conor McDowell Approved: __________________________________________________________ Christopher K. Sommerfield, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee Approved: __________________________________________________________ Mark A. Moline, Ph.D. Director of the School of Marine Science and Policy Approved: __________________________________________________________ Mohsen Badiey, Ph.D. Acting Dean of the College of Earth, Ocean, and Environment Approved: __________________________________________________________ Ann L. Ardis, Ph.D. Senior Vice Provost for Graduate and Professional Education ACKNOWLEDGMENTS This research was funded by National Fish and Wildlife Foundation grant #43752 to Dr. J.T. Kirby, University of Delaware. Additional support to the author was provided by the Marian R. Okie Fellowship and the University of Delaware. The author would like to thank his advisor, Dr. Christopher Sommerfield, for all of his support and mentorship. He would also like to thank his committee members Dr. James Pizzuto and Dr. James Kirby for their guidance and input. Kaitlin Tucker, Katherine Pijanowski, Brandon Boyd, and Daniel Duval, were essential in both the lab and the field. Collaborators at the USFWS made this research possible. The author would like to thank Susan Guiteras, Laura Mitchell, Rodrick Murray, Collin Brown, and Curtis George at Bombay Hook NWR for their support. iii TABLE OF CONTENTS LIST OF TABLES ........................................................................................................ vi LIST OF FIGURES ...................................................................................................... vii ABSTRACT ................................................................................................................ viii Chapter 1 INTRODUCTION .............................................................................................. 1 2 BACKGROUND ................................................................................................ 4 2.1 Marsh Accretion and Elevation Change .................................................... 4 2.2 Processes that Control Marsh Boundaries ................................................. 5 3 STUDY AREA ................................................................................................... 8 3.1 Geologic Setting ........................................................................................ 8 3.2 Wetland Vegetation and Hydrology ........................................................ 10 3.3 Human Intervention ................................................................................. 11 4 METHODS ....................................................................................................... 14 4.1 Analysis of Marsh Topography ............................................................... 14 4.2 Core Collection ........................................................................................ 15 4.3 Physical Property Measurements ............................................................ 16 4.4 Radionuclide Geochronology .................................................................. 17 4.5 Accretion and Accumulation Rates ......................................................... 18 5 RESULTS ......................................................................................................... 20 5.1 Marsh Topography and Spatial Distribution ........................................... 20 5.2 Marsh Soil Physical Properties ................................................................ 21 5.3 Radionuclide Activity Profiles and Inventories ...................................... 23 5.4 Marsh Accretion and Accumulation Rates .............................................. 24 6 DISCUSSION ................................................................................................... 26 6.1 Marsh Accretion Rates and Relative Sea-level Rise ............................... 26 6.2 Spatial Variation in Accretion and Accumulation Rates ......................... 26 6.3 Sediment Production by Marsh Edge Erosion ........................................ 28 6.4 Spatial Variability of Salt Marsh Platform Loss ..................................... 30 6.5 Salt Marsh Loss and Estuarine Transgression ......................................... 33 7 CONCLUSIONS .............................................................................................. 36 iv TABLES ....................................................................................................................... 39 FIGURES ..................................................................................................................... 43 REFERENCES ............................................................................................................. 58 APPENDICIES A MARSH CORE PHYSICAL PROPERTY DATA ................................. 67 B MARSH CORE RADIONUCLIDE DATA ............................................ 81 v LIST OF TABLES Table 1: Core locations, elevations, dominant vegetation type, and core- averaged dry bulk density. ....................................................................... 39 Table 2: Change in salt marsh platform area between 1961 and 2012. ................. 40 Table 3: Radionuclide inventories, mineral sediment and organic matter accumulation since 1963, and accretion rates for core sites at Bombay Hook NWR. ............................................................................................. 41 Table 4: Calculation of mineral sediment gained through mineral sediment accumulation and lost through marsh platform erosion. ......................... 42 vi LIST OF FIGURES Figure 1: Conceptual model of feedbacks among tidal hydrodynamics, sediment supply, ecological processes, and marsh elevation gain. ........................ 43 Figure 2: Location of Bombay Hook NWR within the Delaware Estuary ............. 44 Figure 3: Map of salt marshes zones at the refuge based on dominant vegetation. ............................................................................................... 45 Figure 4: LiDAR elevation map of Bombay Hook NWR. ..................................... 46 Figure 5: Hypsometric curves for the refuge based on analysis of LiDAR topography. .............................................................................................. 47 Figure 6: Areal extent of the Bombay Hook NWR salt marsh in 1961 and 2012 and classified areas of marsh gain and loss between 1961 and 2012. ..... 48 Figure 7: Depth profiles of sediment dry bulk density for the marsh cores. .......... 49 Figure 8: Depth profiles soil volume composition of the marsh cores. .................. 50 Figure 9: Depth profiles of 137Cs activity for the marsh cores. ............................... 51 Figure 10: Depth profiles of supported 210Pb and total 210Pb for the marsh cores. ... 52 210 Figure 11: Depth profiles of LN( Pbxs Activity) used to calculate accretion rates 53 Figure 12: Cesium-137 and 210Pb accretion rates for the marsh coring sites. ........... 54 Figure 13: Lead-210 and 137Cs-derived mass accumulation rates for the marsh cores. ........................................................................................................ 55 Figure 14: Stacked bar plot of mineral sediment and organic matter accumulation at the coring sites since 1963, based in the depth of the 137Cs peak in the marsh soil. .......................................................................................... 56 Figure 15: Scatter plot of 137Cs-derived accretion rate versus organic matter and mineral sediment mass accumulated since 1963. .................................... 57 vii ABSTRACT Bombay Hook National Wildlife Refuge in coastal Delaware protects one of the most expansive salt marsh systems on the U.S. Mid-Atlantic seaboard. In recent decades, the Refuge has experienced a substantial decrease in salt marsh area along the Delaware Bay boundary by shoreface erosion and in the marsh interior by inland pool expansion. Although the origin of the pools is unknown, it has been suggested that the supply of allochthonous mineral sediment from tidal waterways to the marsh platform may be a contributing factor. To investigate whether vertical accretion of Refuge marshland is limited by sediment accumulation, a study was conducted to measure rates of mineral sediment and organic matter accumulation (mass/area/time)