Mass balance implies Holocene development of a low-relief karst patterned landscape

Abstract

We constructed mass balances of both calcium and phosphorus for two watersheds in Big Cypress National Preserve in southwest Florida (USA) to evaluate the time scales over which its striking landscape pattern developed. This low-relief carbonate landscape is dotted with evenly spaced, evenly sized, shallow surface depressions that annually fill with surface water and thus support wetland ecosystems (e.g. cypress domes) embedded in a pine-dominated upland matrix with exposed bedrock. Local and landscape scale feedbacks between hydrology, ecological dynamics and limestone dissolution are hypothesized to explain this karst dissolution patterning. This hypothesis requires the region to be wet enough to initiate surface water storage, which constrains landscape formation to interglacial periods. The time scale therefore would be relatively recent if creation of the observed pattern occurred in the current interglacial period (i.e. Holocene), and older time scales could reflect inherited patterns from previous inter-glacial periods, or from other processes of abiotic karstification. We determined phosphorus stocks across four landscape compartments and estimated the limestone void space (i.e., wetland depression volume) across the landscape to represent cumulative calcium export. We calculated fluxes in (e.g., atmospheric deposition) and out (i.e., solute export) of the landscape to determine landscape denudation rates through mass balance. Comparing stocks and annual fluxes yielded independent estimates of landscape age from the calcium and phosphorus budgets. Our mass balance results indicate that the landscape began to develop in the early-mid Holocene (12,000-5000 ybp). Radiocarbon dating estimates implied similar rates of dissolution (~1 m per 3000-3500 years), and were in agreement with Holocene origin. This supports the hypothesis that ecohydrologic feedbacks between hydrology and vegetation occurring during the present interglacial period are sufficient to shape this landscape into the patterns we see today, and more broadly suggests the potential importance of biota in the development of macro-scale karst features.

Initiation and Development of Wetlands in Southern Florida Karst Landscape Associated With Accumulation of Organic Matter and Vegetation Evolution

“…Our results indicate this approach is more reliable for age dating compared with using other dating materials because of the myriad confounding factors that can impact shallow sediments in this warm carbonate‐dominated environment. Our LCFAs age dates from wetlands in BICY indicate development of the modern patterned landscape in the middle to late Holocene, in agreement with estimates from mass balance modeling (Chamberlin et al, ). During early development, RP sites were likely dominated by woody plants (e.g., cypress trees), while TR sites exhibited a dominance of graminoids, a contrast that likely arose from differences in elevation and hydroperiods.…”

“…BP (Table ). Similar mid‐Holocene ages for initiation of the formation of depressions were found based on calcium and phosphorus mass balance estimates in BICY (Chamberlin et al, ) and theoretical estimates of the time necessary to dissolve bedrock under wetlands to a depth of 2 m (Dong et al, ). McDowell et al () reported that peats were deposited right above the limestone in the northern Everglades at ~4,280 BP.…”

“…We sought to test hypotheses based on an assumption, evaluated in the project, that LCFAs provide better 14 C age record of wetland development than other organic carbon (OC) sources. Assuming this assumption is correct, we test the hypotheses that (1) landscape patterning developed in the middle to late Holocene, as suggested by Dong et al () and Chamberlin et al (); (2) vegetation changes in the historical landscape were driven by precipitation and local hydrological conditions; (3) soil OM in the center of the domes plays a more important role in bedrock weathering because prolonged inundation reduces CO 2 ventilation and increases carbonic acid production at the water‐bedrock interface compared with the edges.…”

“…The origins of the observed self‐organizing patterning are not well understood (Dong et al, ; Watts et al, ) but likely relate to interactions between OM burial, remineralization processes creating carbonic acid, and carbonate mineral dissolution and precipitation. Key uncertainties include the age of the extant landforms (e.g., Chamberlin et al, ) and the internal OM dynamics that are thought to control formation of the patterning.…”

Scale‐Dependent Patterning of Wetland Depressions in a Low‐Relief Karst Landscape

“…The rapid decline in probabilities of larger basins suggests that larger, more established basins have ceased to expand laterally. Given the recent origins of the landscape pattern (Chamberlin et al, ) and the long time scales necessary for carbonate dissolution (Dong et al, ), this interpretation is tenable and leads to the prediction that dissolution rates, and thus lateral expansion, are variable with wetland size.…”