Spatio-temporal Dynamics of Soil Composition and Accumulation Rates in Mangrove Wetlands

Coastal wetlands are globally important environments for biogeochemical cycling and are the object of intensive research related to the sequestration and exchange of carbon with oceans, continents, and the atmosphere. Wetland soil core records of organic carbon (OC) provide insights about future ecosystem responses to global change by identifying temporal variability in the context of environmental changes including sea level rise (SLR), anthropogenic reductions in freshwater flow, and landscape-scale disturbance events. My studies of Gulf of Mexico mangroves involved the use of radiometrically-dated soil cores to identify spatial and temporal accumulation trends of various constituents including organic and carbonate carbon, and macro-nutrients. My dissertation includes a literature review to assess the timescales of these processes and refine global perspectives on coastal wetland vulnerability. The contributions of organic and mineral matter to soil accretion (mm yr-1) was measured to (a) quantify how the supply of each may allow regional mangroves to keep pace with various SLR scenarios and, (b) assess wetland carbon sink capacity and stability in southwest Florida and the Yucatan Peninsula of Mexico. Mangroves in this region are largely devoid of terrigenous mineral sediments, and it has been hypothesized that storm surge-driven accretion of marine sediments could improve the capability of these locations to keep pace with SLR. Rates of accretion and organic matter accumulation were statistically similar across all four study regions, whereas mineral deposition rates ranged over two orders of magnitude. The volumetric contribution of mineral sediment to accretion is minimized by its high density. Organic matter, whose porous structures allow for highly variable densities, can contribute to a wide range of accretion rates and is a strong predictor of accretion. Future sustainability of these wetlands is more strongly dependent on the balance between soil organic matter production and preservation than the provision of storm-derived mineral sediments. To understand how OC sequestration will respond to SLR, the spatial and temporal variability of OC burial rates (g m-2 yr-1) were examined across ecosystem gradients in salinity, nutrient availability and mangrove productivity in the coastal Everglades. Results showed relatively little spatial variability and indicated that OC burial in the region is slow compared to rates in mangroves globally. However, significant regional differences in OC burial were observed in the context of primary productivity. Over a centennial timescale, mid-stream sites sequestered roughly 22% of annual net primary production and upstream sites preserved less than 10%. Least efficient sequestration occurs in the oligohaline ecotone, where increases in groundwater salinities and the potential for sulfate reduction have been recorded in the past decade. These findings indicate a significant slowdown in OC burial, and suggest that accelerating SLR will cause a substantial loss of historically sequestered carbon. The loss and potential out-welling of this carbon (including particulate and dissolved organic matter, dissolved CO2, and carbonate alkalinity) has important and complex implications for neighboring marine ecosystems including coral reefs and seagrass meadows. Several recent high-profile publications have used 5–15 years of soil accumulation rates to model wetland SLR-vulnerability outcomes over the next 50–100 years. To provide perspectives on these models, data that were generated from observations on multiple timescales (sub-annual to millennial) around the globe were used in a meta-analysis to determine the role of observational timescale on assessment outcomes. This analysis focused on rates of accretion and elevation change because of the wide availability of these data. Results demonstrate that rates of soil-body change exhibit a dependence on the length of time over which observations are made. Timescale hierarchies are driven by post-depositional diagenesis, ecosystem state changes, and regional effects primarily related to hydrology and sediment supply. Longer periods of observation utilizing multiple geochronological methods are needed to differentiate trend-changes from apparent changes that, in fact, may be due to regular periodicity. A conceptual model is presented that categorizes and explains timescale hierarchies in a soil’s geochemical history

Spatio-temporal Dynamics of Soil Composition and Accumulation Rates in Mangrove Wetlands

Coastal wetlands are globally important environments for biogeochemical cycling and are the object of intensive research related to the sequestration and exchange of carbon with oceans, continents, and the atmosphere. Wetland soil core records of organic carbon (OC) provide insights about future ecosystem responses to global change by identifying temporal variability in the context of environmental changes including sea level rise (SLR), anthropogenic reductions in freshwater flow, and landscape-scale disturbance events. My studies of Gulf of Mexico mangroves involved the use of radiometrically-dated soil cores to identify spatial and temporal accumulation trends of various constituents including organic and carbonate carbon, and macro-nutrients. My dissertation includes a literature review to assess the timescales of these processes and refine global perspectives on coastal wetland vulnerability. The contributions of organic and mineral matter to soil accretion (mm yr-1) was measured to (a) quantify how the supply of each may allow regional mangroves to keep pace with various SLR scenarios and, (b) assess wetland carbon sink capacity and stability in southwest Florida and the Yucatan Peninsula of Mexico. Mangroves in this region are largely devoid of terrigenous mineral sediments, and it has been hypothesized that storm surge-driven accretion of marine sediments could improve the capability of these locations to keep pace with SLR. Rates of accretion and organic matter accumulation were statistically similar across all four study regions, whereas mineral deposition rates ranged over two orders of magnitude. The volumetric contribution of mineral sediment to accretion is minimized by its high density. Organic matter, whose porous structures allow for highly variable densities, can contribute to a wide range of accretion rates and is a strong predictor of accretion. Future sustainability of these wetlands is more strongly dependent on the balance between soil organic matter production and preservation than the provision of storm-derived mineral sediments. To understand how OC sequestration will respond to SLR, the spatial and temporal variability of OC burial rates (g m-2 yr-1) were examined across ecosystem gradients in salinity, nutrient availability and mangrove productivity in the coastal Everglades. Results showed relatively little spatial variability and indicated that OC burial in the region is slow compared to rates in mangroves globally. However, significant regional differences in OC burial were observed in the context of primary productivity. Over a centennial timescale, mid-stream sites sequestered roughly 22% of annual net primary production and upstream sites preserved less than 10%. Least efficient sequestration occurs in the oligohaline ecotone, where increases in groundwater salinities and the potential for sulfate reduction have been recorded in the past decade. These findings indicate a significant slowdown in OC burial, and suggest that accelerating SLR will cause a substantial loss of historically sequestered carbon. The loss and potential out-welling of this carbon (including particulate and dissolved organic matter, dissolved CO2, and carbonate alkalinity) has important and complex implications for neighboring marine ecosystems including coral reefs and seagrass meadows. Several recent high-profile publications have used 5–15 years of soil accumulation rates to model wetland SLR-vulnerability outcomes over the next 50–100 years. To provide perspectives on these models, data that were generated from observations on multiple timescales (sub-annual to millennial) around the globe were used in a meta-analysis to determine the role of observational timescale on assessment outcomes. This analysis focused on rates of accretion and elevation change because of the wide availability of these data. Results demonstrate that rates of soil-body change exhibit a dependence on the length of time over which observations are made. Timescale hierarchies are driven by post-depositional diagenesis, ecosystem state changes, and regional effects primarily related to hydrology and sediment supply. Longer periods of observation utilizing multiple geochronological methods are needed to differentiate trend-changes from apparent changes that, in fact, may be due to regular periodicity. A conceptual model is presented that categorizes and explains timescale hierarchies in a soil’s geochemical history

Organic Carbon Burial Rates in Mangrove Soils: Global Context and A preliminary Investigation of the Coastal Everglades

The ecological and economic contributions made by mangroves have been well documented in recent decades, and these coastal forests have been the focus of increased attention in terms of conservation and restoration. One aspect that has recently drawn increased attention is the role of mangrove environments in the global carbon cycle, particularly for their high burial rates of organic carbon (OC), also known as “Blue Carbon”, that would otherwise contribute to increased atmospheric CO2 levels. Globally, the amount of available data has more than doubled since the last primary literature review of OC burial in mangrove sediments (2003). The objective of this research is to recalculate the centennial-scale burial rate of OC at both the local and global scales. Quantification of this rate enables better understanding of the current carbon sink capacity of mangroves, as well as helping to quantify and/or validate the other aspects of the mangrove carbon budget such as import, export, and remineralization. Our estimate is that mangrove systems bury 163 (+39.2; -32) g OC m-2 yr-1. Globally, the annual burial rate is 26.1 (+6.3; -5.1) Tg OC. This represents 10-15% of estimated annual mangrove production and supports previous conclusions that, on a centennial timescale, 8 to 15% of all OC burial in marine settings occurs in mangrove systems. The second objective of this research is to provide direct measurements of spatiotemporal differences in OC burial rates at a high-productivity mangrove forest near the mouth of the Shark River in Everglades National Park. The burial rate of OC was determined via radiometric dating (i.e. 210Pb) of six soil cores taken at distances ranging from 25 – 170 m from the Shark River. The resulting mean OC burial rate for the site is 124 g m-2 yr-1, considerably lower than the global estimate. When compared with the local production estimate, the OC burial fraction ranges from 8 to 12% over the course of a century. While the accumulation rate of inorganic matter generally decreases with distance from open water, the OC burial rates show much less predictability, indicating the influence of different controlling mechanisms. Additionally, each core demonstrates a signature of influence from hurricane Wilma (2005). The enhancement of both OC burial and soil surface accretion rates offer evidence of positive hurricane impacts that need to be balanced with assessment of the destructive contributions such as tree mortality and erosion

Spatial variability of organic carbon, CaCO3 and nutrient burial rates spanning a mangrove productivity gradient in the coastal Everglades

Mangrove wetlands are some of the most important locations of organic carbon (OC) sequestration and storage in the world on a per area basis. The high stocks of soil OC are driven by generally high burial rates and efficient preservation of organic material over past millennia of relatively slow and consistent sea level rise. Although the global average rate of OC burial in mangrove wetlands is relatively high, the range in the literature varies by up to two orders of magnitude. The objective of this research was to measure burial rates of OC, CaCO3, and nutrients [total nitrogen (TN) and phosphorous (TP)] across a pronounced ecosystem gradient of productivity and salinity in the coastal Everglades of southwestern Florida, USA. Concentrations and burial rates of both CaCO3 (range 13–1233 g m−2 y−1) and TP (range 0.10–1.59 g m−2 y−1) decreased significantly with distance from the Gulf of Mexico. In contrast, there was less spatial variability in OC (134 ± 12 (1 SE) g m−2 y−1) and TN (6.2 ± 0.4 g m−2 y−1) burial rates. However, significant (P \u3c 0.001) regional differences in OC burial rates were observed relative to mangrove primary productivity. Over a centennial timescale, downstream sites buried 14% of annual net primary production, midstream sites buried 22%, and upstream sites preserved less than 10%

Sediment accretion and organic carbon burial relative to sea-level rise and storm events in two mangrove forests in Everglades National Park.

The goal of this investigation was to examine how sediment accretion and organic carbon (OC) burial rates in mangrove forests respond to climate change. Specifically, will the accretion rates keep pace with sea-level rise, and what is the source and fate of OC in the system? Mass accumulation, accretion and OC burial rates were determined via Pb-210 dating (i.e. 100 year time scale) on sediment cores collected from two mangrove forest sites within Everglades National Park, Florida (USA). Enhanced mass accumulation, accretion and OC burial rates were found in an upper layer that corresponded to a well-documented storm surge deposit. Accretion rates were 5.9 and 6.5 mm yr(-1) within the storm deposit compared to overall rates of 2.5 and 3.6 mm yr(-1). These rates were found to be matching or exceeding average sea-level rise reported for Key West, Florida. Organic carbon burial rates were 260 and 393 g m(-2) yr(-1) within the storm deposit compared to 151 and 168 g m(-2) yr(-1) overall burial rates. The overall rates are similar to global estimates for OC burial in marine wetlands. With tropical storms being a frequent occurrence in this region the resulting storm surge deposits are an important mechanism for maintaining both overall accretion and OC burial rates. Enhanced OC burial rates within the storm deposit could be due to an increase in productivity created from higher concentrations of phosphorus within storm-delivered sediments and/or from the deposition of allochthonous OC. Climate change-amplified storms and sea-level rise could damage mangrove forests, exposing previously buried OC to oxidation and contribute to increasing atmospheric CO2 concentrations. However, the processes described here provide a mechanism whereby oxidation of OC would be limited and the overall OC reservoir maintained within the mangrove forest sediments

Temporal variability of carbon and nutrient burial, sediment accretion, and mass accumulation over the past century in a carbonate platform mangrove forest of the Florida Everglades.

The objective of this research was to measure temporal variability in accretion and mass sedimentation rates (including organic carbon (OC), total nitrogen (TN), and total phosphorous (TP)) from the past century in a mangrove forest on the Shark River in Everglades National Park, USA. The 210Pb Constant Rate of Supply model was applied to six soil cores to calculate annual rates over the most recent 10-, 50-, and 100-year time spans. Our results show that rates integrated over longer timeframes are lower than those for shorter, recent periods of observation. Additionally, the substantial spatial variability between cores over the 10-year period is diminished over the 100-year record, raising two important implications. First, a multiple-decade assessment of soil accretion and OC burial provides a more conservative estimate, and is likely to be most relevant for forecasting these rates relative to long-term processes of sea level rise and climate change mitigation. Secondly, a small number of sampling locations are better able to account for spatial variability over the longer periods than for the shorter periods. The site average 100-year OC burial rate, 123 ± 19 (SD) g m-2 yr-1, is low compared with global mangrove values. High TN and TP burial rates in recent decades may lead to increased soil carbon remineralization, contributing to the low carbon burial rates. Finally, the strong correlation between OC burial and accretion across this site signals the substantial contribution of OC to soil building in addition to the ecosystem service of CO2 sequestration

Organic carbon burial rates in mangrove sediments: Strengthening the global budget.

Mangrove wetlands exist in the transition zone between terrestrial and marine environments and as such were historically overlooked in discussions of terrestrial and marine carbon cycling. In recent decades, mangroves have increasingly been credited with producing and burying large quantities of organic carbon (OC). The amount of available data regarding OC burial in mangrove soils has more than doubled since the last primary literature review (2003). This includes data from some of the largest, most developed mangrove forests in the world, providing an opportunity to strengthen the global estimate. First-time representation is now included for mangroves in Brazil, Colombia, Malaysia, Indonesia, China, Japan, Vietnam, and Thailand, along with additional data from Mexico and the United States. Our objective is to recalculate the centennial-scale burial rate of OC at both the local and global scales. Quantification of this rate enables better understanding of the current carbon sink capacity of mangroves as well as helps to quantify and/or validate the other aspects of the mangrove carbon budget such as import, export, and remineralization. Statistical analysis of the data supports use of the geometric mean as the most reliable central tendency measurement. Our estimate is that mangrove systems bury 163 (+40; -31) g OC m(-2) yr(-1) (95% C.I.). Globally, the 95% confidence interval for the annual burial rate is 26.1 (+6.3; -5.1) Tg OC. This equates to a burial fraction that is 42% larger than that of the most recent mangrove carbon budget (2008), and represents 10-15% of estimated annual mangrove production. This global rate supports previous conclusions that, on a centennial time scale, 8-15% of all OC burial in marine settings occurs in mangrove systems

Temporal variability of carbon and nutrient burial, sediment accretion, and mass accumulation over the past century in a carbonate platform mangrove forest of the Florida Everglades

The objective of this research was to measure temporal variability in accretion and mass sedimentation rates (including organic carbon (OC), total nitrogen (TN), and total phosphorous (TP)) from the past century in a mangrove forest on the Shark River in Everglades National Park, USA. The 210Pb Constant Rate of Supply model was applied to six soil cores to calculate annual rates over the most recent 10, 50, and 100 year time spans. Our results show that rates integrated over longer timeframes are lower than those for shorter, recent periods of observation. Additionally, the substantial spatial variability between cores over the 10 year period is diminished over the 100 year record, raising two important implications. First, a multiple-decade assessment of soil accretion and OC burial provides a more conservative estimate and is likely to be most relevant for forecasting these rates relative to long-term processes of sea level rise and climate change mitigation. Second, a small number of sampling locations are better able to account for spatial variability over the longer periods than for the shorter periods. The site average 100 year OC burial rate, 123 ± 19 (standard deviation) g m−2 yr−1, is low compared with global mangrove values. High TN and TP burial rates in recent decades may lead to increased soil carbon remineralization, contributing to the low carbon burial rates. Finally, the strong correlation between OC burial and accretion across this site signals the substantial contribution of OC to soil building in addition to the ecosystem service of CO2 sequestration

Utilizing fossilized charcoal to augment the Everglades National Park Fire History Geodatabase

Everglades National Park (ENP) has been documenting fire events since 1948, and these data have been incorporated into an ESRI geodatabase. According to this geodatabase, 757,078 ha of wetlands burned from 1948 to 2011. The main type of vegetation that has burned is comprised of palustrine and estuarine wetlands. However, there are areas in ENP that are comprised of these wetlands that have no documented fire events. We examined fossil charcoal in soil cores and found evidence that fires did indeed occur in some of these areas. Sites of known fires were used to validate the fossil charcoal method. The abundance of fossil charcoal in soil cores from six locations in ENP was measured. Two of the cores were taken from areas with well-documented fire events and four cores where taken from areas with no documented fire events. Three of the cores were dated using 210Pb geochronology. The initial goal was to determine if fires had gone undetected or undocumented in the geodatabase with the ultimate goal being to demonstrate the usefulness of this approach to augment the geodatabase and therefore enhance our understanding of fire ecology in ENP

Avoiding Timescale Bias in Assessments of Coastal Wetland Vertical Change

There is concern that accelerating sea‐level rise will exceed the vertical growth capacity of coastal‐wetland substrates in many regions by the end of this century. Vertical vulnerability estimates rely on measurements of accretion and/or surface‐elevation‐change derived from soil cores and/or surface elevation tables (SETs). To date there has not been a broad examination of whether the multiple timescales represented by the processes of accretion and elevation change are equally well‐suited for quantifying the trajectories of wetland vertical change in coming decades and centuries. To examine the potential for timescale bias in assessments of vertical change, we compared rates of accretion and surface elevation change using data derived from a review of the literature. In the first approach, average rates of elevation change were compared with timescale‐averaged accretion rates from six regions around the world where sub‐decadal, decadal, centennial, and millennial timescales were represented. Second, to isolate spatial variability, temporal comparisons were made for regionally unique environmental categories within each region. Last, comparisons were made of records from sites where SET‐MH stations and radiometric measurements were co‐located in close proximity. We find that rates vary significantly as a function of measurement timescale and that the pattern and magnitude of variation between timescales are location‐specific. Failure to identify and account for temporal variability in rates will produce biased assessments of the vertical change capacity of coastal wetlands. Robust vulnerability assessments should combine accretion rates from multiple timescales with the longest available SET record to provide long‐term context for ongoing monitoring observations and projections