The Palaeo-Agulhas Plain: Temporal and spatial variation in an extraordinary extinct ecosystem of the Pleistocene of the Cape Floristic Region
Introduction
The Cape Floristic Region (CFR), a relatively small region on a global scale (Allsopp et al., 2014), garners an enormous amount of scientific interest. It contains a mega-diverse and unique flora that has been a continual focus of fascination and study. Concentrated on its current shore are a large sample of caves and rock shelters that contain world-renowned palaeo-archives of early modern human biology, behavior, and culture (Marean et al., 2014; Wadley, 2015). These same caves and rockshelters, along with some open-air localities, preserve extensive records of faunal change that document extraordinary shifts in faunal communities, communities that seem to change more dramatically since the onset of the Holocene than anywhere else in Africa (Klein, 1983). Its oceanic peculiarities, shaped by colliding cold upwelling and warm tropical currents, create a rich two-ocean environment (Branch and Menge, 2001) of intense interest to the public and marine scientists. In addition, its geology preserves rich palaeoarchives that have cultivated a long and distinguished record of Quaternary science.
Despite this ardent scientific gaze, there is a cryptic but crucial feature of the CFR’s character that has remained largely underappreciated and understudied, sometimes forgotten. Scientists have known for half a century that a submerged landscape, most widely expressed on the central southern shores of the CFR (Fig. 1), existed underwater (Dingle, 1970a, b; Slater, 1970). This knowledge failed to translate into appreciation of the potential significance of this ancient ecosystem for virtually all of those things that have drawn that selfsame scientific fascination – floral diversity, massive faunal change, and early complex human behavior and culture. This lost palaeoscape, what we have called the Palaeo-Agulhas Plain (Marean et al., 2014), is now hidden but, because sea levels were predominantly lower through most of the Quaternary, it would have been an important, and we argue pivotal, now extinct ecosystem.
Recently, Carr et al. (2016) opined that “the nature of the vegetation on the continental shelf remains a critical unresolved element for interpretations of both the human and faunal records in this region …” partially due to “the poorly defined nature and distribution of continental shelf substrates” (page 38). In prior papers, we outlined a research agenda and methodology for tackling these palaeoecological puzzles (Franklin et al., 2015; Marean et al., 2015) and offered some preliminary hypotheses of the PAP character (Marean et al., 2014). With those hypotheses, we sketched a biophysical environment that set it apart from the other well-known CFR physiographies, and, therefore, we gave it a formal name – the Palaeo-Agulhas Plain (PAP). We then built a research team that, in this special issue, provides much of the information Carr et al. (2016) lament the lack of, and use it to build a model of that ‘unresolved vegetation’.
In 1979, the seminal Serengeti: Dynamics of an Ecosystem (Sinclair and Norton-Griffiths, 1979) was published, and this groundbreaking synthesis of a migration ecosystem created a science model for future generations aspiring to fathom the complexities of abiotic and biotic interactions on a regional scale. The CFR was subject to similar treatment in 1992 with The Ecology of Fynbos: Nutrients, Fire, and Diversity (Cowling, 1992). Important updates have followed (Allsopp et al., 2014) and now we have deep and sophisticated understandings of these ecosystems. As palaeoscientists, we must aspire to reach such complex understandings of palaeoscapes as our fellow scientists have assembled for their modern ecosystems. Overall, we think the PAP resembled its far northern kin, the Serengeti, more than its neighbor next door, the contemporary CFR.
While we see the interdisciplinary study and understanding of these modern ecosystems as aspirational goals for the palaeosciences, we think we have brought to the table something that perhaps even these modern studies have yet to attain – a nearly fully coupled abiotic-biotic-human behavior computer model of the PAP. Marean et al. (2015) described a research agenda where palaeoscientists would develop linked computer simulation models, constantly under improvement, that would begin with models of land, sea, geology, soil, and climate. The majority of the abiotic characteristics would come from field and laboratory study, while climate would be projected using climate modeling validated using palaeo-archives. We would then employ vegetation modeling (Franklin et al., 2015) to project the flora across this modeled palaeoscape, and again validate those projections with palaeo-archive data. Since our study had a strong paleoanthropological objective, we proposed reconstructing a resourcescape for humans – a resourcescape is a formal model, built on the vegetation model, of the spatial distribution of resources sought by humans (food, water, wood, etc.). Finally, we would employ agent based modeling guided by the principles of behavioral ecology to model the behavior of ancient people in this ecosystem. We would then test the behaviors predicted by the model with the archaeological record. This volume and the papers in it provide some of our first research results from this approach applied to the PAP.
In this paper, we present a model of the PAP’s abiotic and biotic characteristics and changing palaeoecology from marine isotope stage (MIS) 6 through 1 (around 195ka to the present). In so doing, we address the following research questions: 1) What was the ecology of this now extinct coastal ecosystem, and how did it compare to the contemporary coastal forelands of the CFR? 2) How did it vary through time and space? 3) What is its significance for understanding modern plant and animal biogeography? 4) What resources did it provide the early modern humans who exploited it, and how does this help us understand the human origins record? We set the stage for this model with a review of our growing knowledge of the PAP.
Section snippets
Background
As part of a national effort to assemble knowledge of its geological resources and map its offshore topography, South Africa and the University of Cape Town initiated the SANCOR Marine Geology Programme in the mid-1960s. From that work, R.V. Dingle, J. Rogers and others began publishing a series of descriptions and interpretations of the “triangular-shaped continental margin” called the Agulhas Bank. This resulted in a preliminary geological and topographic map of the Agulhas Bank and a
Approach
Our approach to developing a palaeoecology of the PAP, and illustrated in this volume, follows the framework outlined in Franklin et al. (2015) and Marean et al. (2015). Our research plan had as a major part of its goal a better understanding of human adaptations in this region and, therefore, our palaeoecology targeted the development of a resourcescape for hunter-gatherers. That resourcescape would be the basis for a quantitative model for the modern environment of resource return rates
A general model of the PAP during the Last Glacial Maximum
Topographically and edaphically, the PAP comprised a landscape without analogue in the extant CFR. Fig. 3 shows a reconstruction of this palaeoscape. The relief was subdued and the region dominated by deep and relatively fertile soils compared to the CFR. Wide (up to 30 km) and shallow river valleys traversed the entire plain and these broadened as they went seaward. Alluvium that yielded deep and fertile soils filled these valleys. These covered 15 000 km2, an eight-fold increase from today’s
Implications for human foraging adaptations
The current archive records for Pleistocene humans along the Cape south coast are well known for their remarkable records of early modern humans in a changing CFR. These sites (Klasies River, Nelson Bay Cave, Pinnacle Point, and Blombos) all currently sit along the coastline, near the intertidal, but during most of their prehistoric occupation humans occupying those sites would have looked out over the PAP (Fisher et al., 2010). We think these prehistoric people focused the majority of their
Conclusions
At the height of its geographic extent, during strong glacial climates, the PAP added a landmass the size of Ireland to the CFR. During strong interglacial climates, the PAP was submerged and its component ecosystems either disappeared entirely, or shrank to small and fragmented slivers along the steeply rising contemporary coastline. The lower sea levels of the Pleistocene glacial periods exposed the PAP to varying degrees, and the dynamic PAP was a significant factor in the diversification of
Acknowledgements
The authors thank all the members of the project for their outstanding contributions. Cowling acknowledges Nelson Mandela University for generous support. We all acknowledge the generous support of the funding agencies: the National Science Foundation (BCS-0524087, BCS-1138073 and BCS-1460376), Hyde Family Foundations, the Institute of Human Origins (IHO) at Arizona State University, PSIF of Arizona State University, and the John Templeton Foundation to the Institute of Human Origins at Arizona
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