Speleothem preservation and diagenesis in South African hominin sites implications for paleoenvironments and geochronology

Abstract

Plio-Pleistocene speleothems from australopithecine-bearing caves of South Africa have the potential to yield paleoenvironmental and geochronological information using isotope geochemistry. Prior to such studies it is important to assess the preservation of geochemical signals within the calcitic and aragonitic speleothems, given the tendency of aragonitic speleothems to recrystallize to calcite. This study documents the geochemical suitability of speleothems from the principal hominin-bearing deposits of South Africa. We use petrography, together with stable isotope and trace element analysis, to identify the occurrence of primary aragonite, primary calcite, and secondary calcite. This study highlights the presence of diagenetic alteration at many of the sites, often observed as interbedded primary and secondary fabrics. Trace element and stable isotopic values distinguish primary calcite from secondary calcite and offer insights into geochemical aspects of the past cave environment. O. High carbon isotope values in these deposits are associated with the effects of recrystallization and rapid outgassing of CO 2 during precipitation. Mg/Ca and Sr/Ca ratios differ between primary and secondary calcite speleothems, aiding their identification. Carbon and oxygen isotope values in primary calcite reflect the proportion of C 3 and C 4 vegetation in the local environment and the oxygen isotope composition of rainfall. Primary calcite speleothems preserve the pristine geochemical signals vital for ongoing paleoenvironmental and geochronological research.

Speleothem evidence for C3 dominated vegetation during the Late Miocene (Messinian) of South Africa

“…However, unlike other studies on secondary calcite speleothems (e.g. Hopley et al, 2009;Holzkämper et al, 2009), the δ 13 C values in the Hoogland Southern Basal Speleothem are low ( Figures 6 & 10), significantly below that of the Malmani dolomite host rock. There is also no correlation between δ 13 C and Mg/Ca or Sr/Ca ratios.…”

“…While this method produces some sample orientation it only requires a small volume of material for analysis (Cheetham, 2002). Six thin sections were produced from representative samples; speleothem fabrics were identified using previously published speleothem studies (Frisia et al, 2000, Frisia et al, 2002, Hopley et al, 2009, Frisia, 2015. Block samples for palaeomagnetic analysis were oriented in the field using a Suunto Compass and Clinometer.…”

“…However, with multiple possible causes for increased δ 13 C values in these speleothems (e.g. Baker et al, 1997;Hopley et al, 2009), an increased C4 plant proportion cannot be proposed with a high degree of confidence.…”

“…A C C E P T E D M A N U S C R I P T 8 columnar calcite crystals, and curved calcite crystallites belonging to the lattice deformation fabric (Figure 5a,b), as described by Broughton (1983). The lattice deformation fabric occurs due to calcite growth on curved surfaces such as stalactites (Broughton, 1983) and mammillary speleothems (Hopley et al, 2009); mammillary speleothems are common in palaeokarst sequences from the Cradle of Humankind, including Hoogland , and typically form in subaqueous (e.g. cave pool) environments (Hill and Forti, 1997;Polyak et al, 2008).…”

stable isotope setting of Australopithecus sediba at Malapa, South Africa

“…7,12 Furthermore, in dolomitic areas such as the CoH, interpretations of oxygen and carbon isotope values from carbonates are affected by post-depositional diagenetic alteration of metastable aragonite to calcite. 13 Despite the difficulties presented by the setting, cave carbonate studies in the CoH remain important for linking climate and environmental change with hominin evolution. Expansion of stable isotope studies to carbonatecemented cave sediments associated with hominin fossils in the CoH provide a valuable source of information for palaeoclimate studies.…”