Late Holocene Sedimentation and Paleoenvironmental History for the Tidal Marshes of the Potomac and Rappahannock Rivers, Tributaries to Chesapeake Bay

Instrumental tide gauge records indicate that the modern rates of sea-level rise in the Chesapeake Bay more than double the global average of 1.2-1.5 mm yr-1. The primary objective for this study is to establish a relative depositional history for the tidal marshes of the Potomac and Rappahannock Rivers that will help us improve our understanding of processes that influence sedimentation in the proximal tributaries of Chesapeake Bay. Marsh cores were collected from Blandfield Point VA, Tappahannock VA, and Potomac Creek VA. The sedimentary facies include: 1) a lower unit of organic-poor, grey clay with fine sand and silt layers and estuarine foraminifera; and 2) an upper unit of organic-rich clay and peat with abundant brackish to freshwater marsh foraminifera and thecamoebians. AMS 14C dating of bulk marsh sediments yield sedimentation rates at Potomac Creek ranging from 3.04-4.20 mm yr-1 for the past 2500 years. Rates of sedimentation calculated for Blandfield Point indicate 1.37-2.19 mm yr-1 in the basal clays and peat for the past ~3000 years. Foraminiferal census counts indicate a freshening upward trend with a transition from an estuarine Ammobaculites crassus assemblage to a marsh Ammoastuta salsa assemblage with abundant freshwater thecamoebians. The late Holocene history of sedimentation for the marshes indicates that differential compaction, recent land use practices, and climate change have contributed to the resultant freshening-upward environmental trend and variability in sediment accumulation rates between coring sites

A Centennial Record of Paleosalinity Change in the Tidal Reaches of the Potomac and Rappahannock Rivers, Tributaries to Chesapeake Bay

Gravity and push cores from the Potomac and Rappahannock Rivers (Virginia Tidewater) were collected from central and proximal estuarine zones with known seasonal salinity stratification. The lowermost microfossil associations in the cores comprise alternating ostracode populations of Cyprideis salebrosa and Cytheromorpha. This microfossil association gives way to an oligohaline association dominated by the freshwater ostracode Darwinula stevensoni. Stable oxygen isotope values (δ18O) of Rapphannock Cyprideis salebrosa are highly variable ranging between -6.6 to -3.2‰ VPDB. δ18O values for Potomac Cytheromorpha fuscata range from -8.2 to -3.2‰ VPDB. Positive excursions in δ18O values are synchronous with population peaks for both Cyprideis and Cytheromorpha indicative of increased marine influence and/or higher salinities. Microfossil paleoecology coupled with oxygen isotope values record a marked shift towards gradual freshening and deterioration of the salinity structure in the tidal tributaries during the mid-to late 19th century. We attribute these trends to both decadal climate trends and aggressive land use practices in the Chesapeake Bay watershed during the late 19th to middle 20th centuries

Ecological response to hydrological variability and catchment development : Insights from a shallow oxbow lake in Lower Mississippi Valley, Arkansas

The ecological response of shallow oxbow lakes to variability in hydrology and catchment development in large river floodplain ecosystems (RFE) in Arkansas remains largely unknown. Investigating these responses will advance our understanding of ecological evolution of oxbow lakes in response to the major environmental drivers, which will establish baseline conditions required to develop effective management practices for RFE. In this pilot study, we examined the potential of using a dated surface sediment core from Adams Bayou, a floodplain lake located within the Cache-Lower White River Ramsar site in SE Arkansas. Stratigraphic records of diatoms and sediment geochemistry were used to ascertain variation in Adams Bayou's ecological condition. During 1968–2008, in response to hydrological and anthropogenic changes, Adams Bayou's diatom assemblages progressed from predominantly benthic (Gomphonema parvulum and Meridion circulare) to primarily planktonic assemblage (Aulacoseira granulata and Cyclotella meneghiniana), along with a decrease in magnetic susceptibility (k) and % silt. Statistical analyses reveled that during 1968–2000, higher hydrological connectivity and catchment alterations drove Adams Bayou's ecosystem. After 2000, lower hydrological connectivity and increase in cultivation were the major drivers. The potential impact of increasing air temperature was also noted. The shift in Adams Bayou from a connected, clear, mesotrophic state to a relatively isolated, turbid and nutrient enriched state is consistent with regime shift models and highlights its sensitivity to a combination of environmental stresses prevalent in the catchment. Although fluvial systems pose challenges in establishing clear chronologies, oxbow lake sediments can be a effective paleoecological archives. Our work provides clear evidence for the change in the ecological character of this wetland of international significance and flags the need for a wider assessment of water bodies across this site under obligations to the Ramsar Convention

Dry Climate Disconnected the Laurentian Great Lakes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94725/1/eost16649.pd

Climatic history of the northeastern United States during the past 3000 years

Many ecosystem processes that influence Earth system feedbacks – vegetation growth, water and nutrient cycling, disturbance regimes – are strongly influenced by multidecadal- to millennial-scale climate variations that cannot be directly observed. Paleoclimate records provide information about these variations, forming the basis of our understanding and modeling of them. Fossil pollen records are abundant in the NE US, but cannot simultaneously provide information about paleoclimate and past vegetation in a modeling context because this leads to circular logic. If pollen data are used to constrain past vegetation changes, then the remaining paleoclimate archives in the northeastern US (NE US) are quite limited. Nonetheless, a growing number of diverse reconstructions have been developed but have not yet been examined together. Here we conduct a systematic review, assessment, and comparison of paleotemperature and paleohydrological proxies from the NE US for the last 3000 years. Regional temperature reconstructions (primarily summer) show a long-term cooling trend (1000 BCE–1700 CE) consistent with hemispheric-scale reconstructions, while hydroclimate data show gradually wetter conditions through the present day. Multiple proxies suggest that a prolonged, widespread drought occurred between 550 and 750 CE. Dry conditions are also evident during the Medieval Climate Anomaly, which was warmer and drier than the Little Ice Age and drier than today. There is some evidence for an acceleration of the longer-term wetting trend in the NE US during the past century; coupled with an abrupt shift from decreasing to increasing temperatures in the past century, these changes could have wide-ranging implications for species distributions, ecosystem dynamics, and extreme weather events. More work is needed to gather paleoclimate data in the NE US to make inter-proxy comparisons and to improve estimates of uncertainty in reconstructions

Northeast US precipitation variability and North American climate teleconnections interpreted from late Holocene varved sediments

A more thorough understanding of regional to hemispheric hydroclimate variability and associated climate patterns is needed in order to validate climate models and project future conditions. In this study, two annually laminated (varved) sediment records spanning the last millennium were analyzed from Rhode Island and New York. Lamination thickness time series from the two locations are significantly correlated to hydroclimate indicators over the period of instrument overlap, demonstrating their usefulness in reconstructing past conditions. Both records are correlated to climate teleconnection indices, most strongly the Pacific/North American (PNA) pattern, suggesting regional to hemispheric influences on hydroclimate. Such a linkage is interpreted to be due to tropospheric circulation patterns in which positive PNA periods are associated with meridional circulation, leading to the dominance of southern moist air masses in the Northeast United States. Alternatively, the zonal flow over North America associated with negative PNA periods produces dominant dry continental air masses over the region. A composite record from the two locations reveals variability of hydroclimate and atmospheric circulation over the late Holocene and shows similarities to previously published reconstructions of the circumpolar vortex and of the Aleutian Low-pressure system, supporting the hypothesized PNA linkage. The record is correlated to continental-scale droughts, many of which have been reconstructed in the American Southwest. These results demonstrate the PNA’s influence on hydroclimate over North America, and suggest that this teleconnected pattern may have a significant role in continental drought dynamics

Anthropogenic influences on estuarine sedimentation and ecology: Examples from the varved sediments of the Pettaquamscutt River Estuary, Rhode Island

Estuaries and lakes are undergoing anthropogenic alterations as development and industry intensify in the modern world. Assessing the ecological health of such water bodies is difficult because accurate accounts of pre-anthropogenic estuarine/lacustrine conditions do not exist. Sediments preserved in water bodies provide archives of environmental changes that can be used to understand both natural and anthropogenic forcings. Here, a high-resolution, multi-proxy approach is used to reconstruct environmental conditions of the Pettaquamscutt River Estuary, Rhode Island, over the last millennium. Two specific time periods reveal anthropogenic alterations to sediment supply and water-column productivity. First, European land clearance at the end of the seventeenth century caused two decades of increased sediment transport through the watershed and increased primary productivity in the water column. Turbidity increases associated with increased water column biomass likely limited green sulfur bacteria that reside below the oxycline. The second anthropogenic effect began in the 1950s with increased residential development in the watershed. Evidence of resulting cultural eutrophication is apparent in both stable nitrogen isotope values as well as in productivity proxies. This effect is likely related to residential development in the watershed and the use of septic systems to treat human waste. These results demonstrate that true base-line conditions of the estuary have not occurred for over three centuries, and that anthropogenic effects can last on the order of decades. The Pettaquamscutt River record serves as a model for using high-resolution sediment records to better understand anthropogenic forcing to natural estuarine/lacustrine systems. © 2008 Springer Science+Business Media B.V

Subdecadal to multidecadal cycles of Late Holocene North Atlantic climate variability preserved by estuarine fossil pigments

The climate system in the North Atlantic region is complex and influenced by outside forcings as well as internal modes of the system. Modeling and observational work have suggested that a better understanding of the connections between ocean- and atmosphere-driven variability could lead to predictive power for North American and European weather patterns. Here we present a new millennial-length proxy record of estuarine fossil pigments and use it to investigate cyclic components of North Atlantic climate through the effects on estuarine ecosystems. The time series exhibits significant cyclic components that can be related to two of the dominant internal modes of climate variability in the region: the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO). The NAO signal is associated with internal atmospheric variability, while the AMO has been linked to previously modeled and observed changes in thermohaline circulation and meridional heatflux. In our record, the dominant periodicity of the AMO has shifted over time, in concert with Medieval Warm Period-Little Ice Age-Present Warm Period transitions. A relationship between an intermittent NAO cycle and the AMO signal suggests coupling of the ocean-atmosphere system at multidecadal time scales. Although the causal relationship is not resolved, predictive models of Northern Hemisphere interannual weather patterns and estuarine productivity may be improved by incorporating the results of this study. © 2006 Geological Society of America

Holocene changes in algal abundance and dissolved oxygen discovered in Sluice Pond, MA through spectroscopy and analysis of a sediment core

Throughout the Holocene, Sluice Pond (Lynn, MA, USA), has experienced fluctuations in algal communities and dissolved oxygen (DO) content based on measurements from a sediment core raised from its anoxic central basin. The variability in the core’s composition can be mapped with Visible Derivative Spectroscopy based on varimax-rotated, principal component analysis (VPCA) through wavelet analysis and by plotting this information against an AMS 14C constrained age model. The temporal history yields a better understanding of the lake’s changing environment and provides insight into the extent of preserved natural and human events. Thirteen separate constituents were present in the core, as mixtures of six different orthogonal (or independent) VPCA components that account for 97.1% of the variance in the data set. Six components were extracted overall, but a detailed look at two is presented in this project. Through the data collected in 6VPCA1 and in 6VPCA6, algal blooms and lake turbidity can be mapped out and referenced against the age model to show changes in relative concentration. The data shows anoxic conditions through the increase or decrease of DO indicators. The first component oscillates on a period of 4 ka, and the second has a 6 ka oscillation. Major climate events such as the Younger Dryas and the 8.2-kiloyear event are represented in the data by a large drop in algal concentrations and an increase in DO during both extreme cold events. Within the last 200 years, fluctuations in algal blooms, turbidity, and DO have increased dramatically in both frequency and extent. Through the data and methods used in this project, we are given a representation of the natural variance over the Holocene and can start to understand how humans may have impacted Sluice Pond. This new information allows for a better understanding of the conditions Sluice Pond has experienced in the past and can inform us on steps that need to be taken for the overall health of the lake.