The Early Miocene Critical Zone at Karungu, Western Kenya: An Equatorial, Open Habitat with Few Primate Remains

Abstract

Early Miocene outcrops near Karungu, Western Kenya, preserve a range of fluvio-lacustrine, lowland landscapes that contain abundant fossils of terrestrial and aquatic vertebrates. Primates are notably rare among these remains, although nearby early Miocene strata on Rusinga Island contain a rich assemblage of fossilized catarrhines and strepsirrhines. To explore possible environmental controls on the occurrence of early Miocene primates, we performed a deep-time Critical Zone (DTCZ) reconstruction focused on floodplain paleosols at the Ngira locality in Karungu. We specifically focused on a single stratigraphic unit (NG15), which preserves moderately developed paleosols that contain a microvertebrate fossil assemblage. Although similarities between deposits at Karungu and Rusinga Island are commonly assumed, physical sedimentary processes, vegetative cover, soil hydrology, and some aspects of climate state are notably different between the two areas. Estimates of paleoclimate parameters using paleosol B horizon elemental chemistry and morphologic properties are consistent with seasonal, dry subhumid conditions, occasional waterlogging, and herbaceous vegetation. The reconstructed small mammal community indicates periodic waterlogging and open-canopy conditions. Based on the presence of herbaceous root traces, abundant microcharcoal, and pedogenic carbonates with high stable carbon isotope ratios, we interpret NG15 to have formed under a warm, seasonally dry, open riparian woodland to wooded grassland, in which at least a subset of the vegetation was likely C 4 biomass. Our results, coupled with previous paleoenvironmental interpretations for deposits on Rusinga Island, demonstrate that there was considerable environmental heterogeneity ranging from open to closed habitats in the early Miocene. We hypothesize that the relative paucity of primates at Karungu was driven by their environmental preference for locally abundant closed canopy vegetation, which was likely absent at Karungu, at least during the NG15 interval if not also earlier and later intervals that have not yet been studied in as much detail.

Reconstruction of the Early Miocene Critical Zone at Loperot, Southwestern Turkana, Kenya

“…Per Lukens et al (2017), calculation of MAT using PWI values is best suited for forested Inceptisols, Alfisols, and Ultisols. PWI values are known to decrease if paleosols are alkaline and/or poorly formed, which can produce calculated MAT values that are unrealistically low.…”

“…To estimate mean annual precipitation (MAP), we calculated a value of hydrolysis for the paleosol units. The ratio of mobile cations to immobile cations within a soil profile can be a proxy for rainfall (e.g., Lukens et al, 2017). The reasoning behind this calculation is that high values of hydrolysis (>1) indicate an abundance of mobile cations in the soil, which implies aridity (i.e., mobile cations are still present within the soil and have not been leached by percolation).…”

“…CIA-K values for Unit 13 ranged from ∼37 (at 0.5 m) to ∼60 (at >1.5 m) (Supplemental Data Table S1). Taking these suggestions, we used CIA-K values for Unit 13 from 0.5 m and 1.0 m depth (within the upper B-horizon), and calculated MAP values ranging from 462 to 545 mm/year (average = 503 mm/yr ± 180 mm) (Supplemental Data Table S1), indicating a semiarid (250-500 mm/yr) to subhumid (500-1000 mm/yr) environment (Bull, 1991;Lukens et al, 2017). We realize that this may not be an estimate of MAP over the entire period of sediment deposition (Adams et al, 2011).…”

“…Units 12, 9, and 7 had chroma values of 3 and suggest a more subhumid climate. Unit 13 values for CIA-K (from 0.5 m and 1.0 m depth, within the upper B-horizon) produced calculated MAP values ranging from 462 to 545 mm/year (average = 503 mm/yr ± 180 mm) (Supplemental Data Table S1), indicating a semiarid (250-500 mm/yr) to subhumid (500-1000 mm/yr) environment (Bull, 1991;Lukens et al, 2017).…”

“…Geographic location (Grossman et al, 2014) as well as varying environmental conditions among regions (affected by climate, tectonics, etc.) certainly helped shape mammal assemblage structure (Andrews and O'Brien, 2010;Lukens et al, 2017). Understanding the influence of each of these biotic and abiotic factors on early Miocene primate distribution and abundance requires analysis of the Critical Zone (Brantley et al, 2007;Nordt and Driese, 2013;Lukens et al, 2017) through a synergistic investigation of geology and paleontology.…”