Quantification of climate change for the last 20,000years from Wonderkrater, South Africa: Implications for the long-term dynamics of the Intertropical Convergence Zone

Abstract

Highlights Quantitative estimates of climate at a key South African site over the past 20 kyr. Reconstructed trends show strong links with regional sea-surface temperatures. Reconstructions indicate interhemispheric synchrony in the African tropics. Findings do not support the hypothesis of direct insolation forcing of tropical African climates. Highlights a promising technique for analyzing palaeobotanical data in the region.

CREST (Climate REconstruction SofTware): a probability density function (PDF)-based quantitative climate reconstruction method

“…Many different strategies have been proposed (Kühl et al, 2002;Gebhardt et al, 2007;Truc et al, 2013), all of them fitting a PDF to the pollen types identified in the fossil record. This strategy leads to a loss of certain information because (1) individual signals are mixed and (2) rare species are masked by the most extended ones.…”

“…They rely on the fundamental hypothesis that a causal relationship exists between the modern distributions of plants and the associated climates (Jackson and Williams, 2004, and references therein). These techniques can be divided into two types: (1) those based on plant assemblages -modern analogue technique (MAT) (Overpeck, 1985;Guiot, 1990), weighted averaging (WA) (ter Braak and Looman, 1986;ter Braak and van Dame, 1989;Birks et al, 1990), weighted averaging-partial least-squares regressions (WA-PLS) (ter Braak and Juggins, 1993), artificial neural networks (ANNs) (Malmgren et al, 2001) or regression trees (Salonen et al, 2012) -and (2) those based on plant distributions -mutual climatic range (MCR) (Atkinson et al, 1987;Sinka and Atkinson, 1999;Elias, 1997), the coexistence approach (CA) (Mosbrugger and Utescher, 1997;Utescher et al, 2014) or probability density functions (PDFs) (Kühl et al, 2002;Truc et al, 2013). These methods are fully detailed in Birks et al (2010).…”

“…These methods are based on the correlation between plants' modern geographical distribution and climate gradients, with the climate value that is the most common in the plant distribution being its "optimum". Among the approaches that have already been proposed within the last decade (Kühl et al, 2002;Gebhardt et al, 2007;Truc et al, 2013), a recurrent issue concerns the assumptions made about the morphological characteristics of the envelope (width, skewness, central tendencies). Kühl et al (2002) fitted a multidimensional Gaussian surface that excluded both multimodality and asymmetry, which are however common features when dealing with botanical assemblages.…”

“…Kühl et al (2002) fitted a multidimensional Gaussian surface that excluded both multimodality and asymmetry, which are however common features when dealing with botanical assemblages. Later, Gebhardt et al (2007) proposed to fit mixture models (combination of several Gaussian surfaces) to relax the constraints of a unimodal Gaussian shape, and more recently Truc et al (2013) proposed the application of non-parametric PDFs to improve the fit between PDF and data.…”

“…However, in areas where pollen types can comprise a high number of plant species ( > 30), it becomes increasingly significant and can result in saturated PDFs. Truc et al (2013) proposed a species selection method (SSM) that recursively alters the taxonomic composition of a pollen type by taking into account the coexistence with other pollen types. In order to minimize PDF saturation, the SSM removes species that have climate requirements that are different from that of the assemblage.…”