Climate records for the early Pleistocene in eastern Africa aid reconstruction of the ecology and landscapes occupied by hominins. Environmental changes associated with alternating wet/dry cycles during this critical interval in hominin evolution are recorded in isotopic profiles from lake sediments from Olduvai Gorge. The Olduvai Gorge Coring Project targeted the depocenter of Paleolake Olduvai, recovering a stratigraphic succession between the Bed I Basalt (~1.90 Ma) and Tuff IF (~1.80 Ma) datums that span hominin fossil horizons in nearby outcrops. Lacustrine claystones from the lower part of this interval are characterized by high Corg (avg. 2.5 %). They display variations in δ13Corg reflecting temporal changes in organic matter (OM) sources. Stratigraphic profiles for n-alkane δ13C values derived from plant waxes are consistent throughout wet/dry cycles indicating stability in savanna vegetation, refuting evidence for increased C4 grasses during drier intervals. δ13C values for n-alkanes derived from aquatic macrophytes and for algal biomarkers increase during drier episodes, reflecting use of bicarbonate as a carbon source triggered by enhanced lake alkalinity. δ13Corg profiles reflect variations in inputs of terrestrial vs. aquatic OM during wet/dry cycles, which are mirrored by δ13C values for hop-17(21)-ene derived from heterotrophic bacteria utilizing sedimentary OM. The δ13C records from Paleolake Olduvai sediments show temporal changes in OM sources rather than shifts in the proportion of C3 and C4 vegetation, indicating that precession-driven climate cycles primarily affected lake environments and associated aquatic resources. Thus, intervals of high climate variability did not necessarily cause changes in savanna vegetation affecting hominin habitats.

Climate records for the early Pleistocene in eastern Africa aid reconstruction of the ecology and landscapes occupied by hominins. Environmental changes associated with alternating wet/dry cycles during this critical interval in hominin evolution are recorded in isotopic profiles from lake sediments from Olduvai Gorge. The Olduvai Gorge Coring Project targeted the depocenter of Paleolake Olduvai, recovering a stratigraphic succession between the Bed I Basalt (~1.90 Ma) and Tuff IF (~1.80 Ma) datums that span hominin fossil horizons in nearby outcrops. Lacustrine claystones from the lower part of this interval are characterized by high C org (avg. 2.5 %). They display variations in δ 13 C org reflecting temporal changes in organic matter (OM) sources. Stratigraphic profiles for n -alkane δ 13 C values derived from plant waxes are consistent throughout wet/dry cycles indicating stability in savanna vegetation, refuting evidence for increased C 4 grasses during drier intervals. δ 13 C values for n -alkanes derived from aquatic macrophytes and for algal biomarkers increase during drier episodes, reflecting use of bicarbonate as a carbon source triggered by enhanced lake alkalinity. δ 13 C org profiles reflect variations in inputs of terrestrial vs. aquatic OM during wet/dry cycles, which are mirrored by δ 13 C values for hop-17(21)-ene derived from heterotrophic bacteria utilizing sedimentary OM. The δ 13 C records from Paleolake Olduvai sediments show temporal changes in OM sources rather than shifts in the proportion of C 3 and C 4 vegetation, indicating that precession-driven climate cycles primarily affected lake environments and associated aquatic resources. Thus, intervals of high climate variability did not necessarily cause changes in savanna vegetation affecting hominin habitats.

The Olduvai Gorge Coring Project (OGCP) recovered sequences of Pleistocene lake sediments that correlate with those known from outcrops directly associated with hominin fossil horizons. The sedimentary succession from Core 2A (02 degrees 58' 43 '' S, 035 degrees 19' 25.5 '' E), which targeted the ancient lake-basinal depocenter, includes stratigraphic intervals (similar to 1.86 Ma) sandwiched between the Bed I basalt and Tuff IF dated horizons, where remains of two hominin holotypes have been recovered. The lower part of this sequence (72.3-86.9 mbs) consists of lacustrine claystones interbedded with sandy claystones characterized by high C-org (av. 2.5%) and includes several laminated intervals. It displays shifts in terrestrial vegetation recorded by values for delta C-13(TOC) and delta 2H(nC)(31) that reflect changes in the proportions of C-3 vs C-4 plants and precipitation, respectively. These temporal profiles represent higher-resolution signatures of the precession-scale wet/dry cycles previously recognized in the outcropping sedimentary succession of Olduvai Gorge. Several samples from this interval contain alkenones comprising C-37 and C-39 alkadien-2-ones and alkatrien-2-ones, and C-38 and C-40 alkadien-3-ones and alkatrien-3-ones. They are distinguished from the vast majority of alkenone distributions reported in lake sediments from polar and temperate latitudes (e.g., Greenland, N. and S. America, Europe, Russia, China, S. Africa, Antarctica) by the absence of tetraunsaturated alkenones (e.g., C-37:4). The alkenone distributions vary stratigraphically with higher concentrations in the drier intervals of the precession-scale cycles that exhibit characteristics resembling those of Group II haptophytes found in contemporary saline/alkaline environments rather than those of Group I haptophytes from freshwater ecosystems. Paleolake Olduvai represents the first reported occurrence of alkenones in a low-latitude Pleistocene lake, where the absence of alkatetraenones may reflect contributions from strains of Group II haptophytes that are favored by warmer temperatures or saline conditions. Temporal variations in unsaturation indices (U-3(7)K', U-3(8)K') and the proportions of alken-2-ones and alken-3-ones likely reflect influences on alkenone production by haptophytes that are associated with changes in the lake environment primarily driven by hydrologic cycles (e.g., salinity and alkalinity) rather than lake temperatures.

Olduvai Gorge in northern Tanzania contains a fossiliferous, well-characterized Pleistocene sedimentary record and provides the opportunity to study the relationships between a changing climate, ecology, and hominin evolution. The Olduvai Gorge Coring Project drilled four cores (1A, 2A, 3A, and 3B) into the depocenter of Paleolake Olduvai in 2014 to achieve increased temporal resolution of local climate and ecological data, and investigate the influence and timing of regional climate and tectonics on local signals. We present high-resolution records of bulk organic carbon isotopes (delta C-13(org), parts per thousand) from Cores 2A and 3A, total organic carbon (wt%) from Cores 2A and 3A, and organic carbon-nitrogen ratios (C:N) from Core 2A. Previous work at Olduvai linked % TOC and delta C-13(org), to orbitally paced variations in lake depth and ecosystem dynamics from Upper Bed I and Lower Bed II (1.9-1.7 Ma), associated with eccentricity maxima and the presence of a perennial saline-alkaline lake in the basin. Bulk organic geochemical properties in both cores exhibit marked shifts in variance and magnitude at 1.9 and 1.7 Ma. Low % TOC values prior to and following 1.9-1.7 Ma implicate low productivity and/or increased degradation of organic matter, while C:N ratios from Core 2A reflect increased aquatic or bacterial input. Within the 1.9 to 1.7 Ma interval, high % TOC is dominated by terrestrial inputs as evidenced by high C:N ratios, and bulk delta C-13(org) captures high variability C3-C4 ecosystem dynamics. Climate variability is highest from 1.9-1.7 Ma, but the delta C-13(org) records are not consistent between Core 2A and Core 3A. From 1.9-1.7 Ma, Core 3A has increased indicators of erosion relative to Core 2A, suggesting a sedimentary aliasing of the delta C-13(org) record in Core 3A. Outside of the 1.9-1.7 Ma interval, both changes in organic carbon source and preferential preservation of proxies for wet conditions may obscure true climate variability, inviting further investigation into the ability of the Olduvai Gorge sedimentary record to test climate variability hypotheses for hominin evolutionary events.

Sediment cores recovered by the Olduvai Gorge Coring Project (OGCP) in 2014 afford the opportunity to examine the coupling of biogeochemical evidence for climatic and environmental change within the context of hominin evolution at this renowned East African locality. Investigations of elemental (total organic carbon - TOC or C-org wt%, carbon to nitrogen ratio - C/N), molecular (source-specific biomarkers), and isotopic (delta C-13(org), delta H-2(nC31)) compositions of organic matter provide evidence for temporal changes in sedimentary materials derived from terrestrial plants (C-3, C-4) and aquatic producers (algae, sponges, cyanobacteria), and in precipitation. The 13 kyr record of Upper Bed I immediately preceding Tuff IB extends high-resolution stratigraphic profiles of precession-scale alternations of wetter and drier conditions to encompass the entire interval from the Bed I Basalt to Tuff IB. The second wetter interval, designated W2, records local influences on climate and water supply consistent with evidence for discrete sediment sources based its physical properties (e.g., gamma radiation, magnetic susceptibility) compared with the overall sequence. The delta C-13(TOC) values for wetter interval W2 reveal two millennial-scale (similar to 2.5 kyr) drier episodes followed by a shift in the dominance of C-3 over C-4 plants accompanying the transition to a drier climate (D3). Moreover, biogeochemical data for Upper Bed I show that changes from drier to wetter conditions occur more rapidly (similar to 900 yr) than wetter to drier transitions ( > 2.6 kyr), based on interpolated ages. In addition, biomarker profiles indicate that aquatic plants, primarily algae and macrophytes, may have been subject to more profound and faster fluctuations than variations in terrestrial vegetation expressed in terms of the relative proportions of woodland and grassland settings. Thus, environmental and climatic changes not only influenced the availability of resources of food and shelter for hominins within the Olduvai region but also led to their variation on centennial to millennial to precessional timescales.

Several hypotheses invoke climatic variability as a driving force for hominin evolution. Thus, high-resolution records of climate and environmental variability from anthropologically significant locations can help test these hypotheses. Sedimentary sequences recovered by the Olduvai Gorge Coring Project (OGCP) help evaluate climatic and environmental changes at Olduvai Gorge, Tanzania through the analyses of various biogeochemical proxies. The stratigraphic sequence of OGCP Core 2A can be correlated with horizons associated with hominins and is chronologically constrained by distinctive dated horizons, such as the Bed I Basalt and Tuff IB. The lacustrine interval from 76.6 to 86.9 m depth is ideally suited for high-resolution analyses of biogeochemical proxies as it is rich in organic carbon (> 1%TOC). The hydrogen isotopic composition of nC(31) in this interval of OGCP Core 2A records the effects of precession-driven wet-dry cycles on the terrestrial environment that led to alternations between woodland and grassland ecosystems, comparable to those documented by previous investigations of outcrop analogues at Olduvai. Here, we examine stratigraphic variations in the abundance of biomarkers (C-28 steradienes, C-17 and C-23 n-alkanes, C-27 and C-28 A-norsteranes, fern-8-ene, and chromans) derived from aquatic organisms (algae, cyanobacteria, sponges, macrophytes, etc.) to determine the response of Paleolake Olduvai to climate variability. In general, these aquatic biomarkers reflect the productivity of the lake environment and exhibit the same precession-driven wet-dry cycles recorded by terrestrial biogeochemical signatures. However, they also provide evidence of abrupt (< similar to 300 yr) changes in lake level and corresponding aquatic communities superimposed on the longer-term Milankovitch cycles. Thus, evidence for climatic variability is manifested through the pacing and intensity of changes in both terrestrial and aquatic ecosystems, but the different rates of the responses on land and in the aquatic environment potentially had a complex influence on water and food resources that were important factors for hominin habitation and evolution.

Olduvai Gorge is renowned for discoveries of hominin fossils and tools in a well-resolved sedimentary context, representing one of the foremost sites in East Africa that has afforded critical evidence of hominin evolution. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered the first deep sediment cores from this location. These cores provide the first opportunity to examine an extensive stratigraphic record from one location and independent of the weathering and related degradation typical of outcrop samples. Samples from these cores have been correlated to Bed I tuffs and basalt marker beds that are well characterized from outcrops within the gorge. This study focuses on the biogeochemical investigation of a similar to 10 m segment from OGCP sediment core 2A, where age constraints indicate deposition occurred over similar to 50,000 years (from 1.8055 +/- 0.003 Ma to 1.8551 +/- 0.013 Ma). The segment includes Tuff IB and IF, which were deposited at a time that prior research indicates represents marked changes in the paleoenvironment at Olduvai, with indications that Paleolake Olduvai may have been completely desiccated by the time Tuff IF was deposited. This environmental change is recorded by the composition of sedimentary organic matter (OM) in terms of bulk organic geochemical properties observed as a decrease in % TOC and delta C-13(org). In addition, temporal variations in source-specific biomarkers (n-alkanes, phytane, steradienes, A-norsteranes and alkenones) reflect a shift from a deeper lake setting with a prevalence of aquatic-sourced OM combined with input from terrestrial plant waxes to a more shallow, perennial lake where evidence of burrows and erosional surfaces is associated with microbially degraded OM and intermittent aquatic biomarkers. Thus, variations in biogeochemical proxies complement the sedimentological evidence confirming increased periods of prolonged desiccation of Paleolake Olduvai beginning around the time of Tuff IB deposition.

Over the past century, numerous discoveries throughout East Africa have advanced our understanding of hominin evolution and provided substantive evidence that climatic and environmental variability played a critical role in evolutionary developments. Stratigraphic records with high temporal resolution aid in testing evolutionary hypotheses that invoke changes in climate and environment at various timescales as drivers of hominin evolution. Olduvai Gorge, Tanzania has a rich history of hominin fossil discoveries from its similar to 2.0 Ma sedimentary sequence, which includes multiple deep lake intervals. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered a sequence of sediment cores that provide an extensive record of the Pleistocene sedimentary history, including paleolacustrine systems in this area. With > 94% core recovery, these cores are ideally suited for high-resolution analyses of organic geochemical proxies and provide an exceptional opportunity to build on previously published outcrop-based paleoenvironmental data from Olduvai. The OGCP core 2A section between 76.6 and 86.9 m depth is considered as a time-stratigraphic equivalent of strata that host the hominin fossils OH24 and OH56 at the Olduvai DK archaeological site. This depositional interval age-bracketed between similar to 1.88 Ma (Bed I Basalt) and similar to 1.85 Ma (Tuff IB) was sampled for organic geochemical analyses. The carbon isotopic composition of organic matter (delta C-13(roc)) from this section varies between values representative of more kforested and open grassland ecosystems over similar to 21 kyr. This observation is consistent with the Milankovitch precession cycle driving the availability of water, which was previously observed in lower resolution studies of outcrop samples. Complementary organic geochemical proxies provide further evidence of these shifts in environmental conditions and record sub-Milankovitch scale changes superimposed on the precession cycle. This suggests the occurrence of short-term fluctuations in the environments inhabited by hominins, which opens new lines of investigation on how environmental scenarios may affect particular evolutionary mechanisms that drove various evolutionary responses.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are the basis of the MBT and CBT indices used as proxies for past changes in temperature and soil pH. Although these compounds were initially thought to originate solely from anaerobic soil bacteria, there is now convincing evidence that they are also produced within aquatic environments. However, the specific source organism(s) remain unknown. We utilize here spooling wire microcombustion-isotope ratio mass spectrometry (SWiM-IRMS) to measure the delta C-13 composition of intact brGDGTs from a series of lake sediments and soils, including samples exhibiting in situ production of brGDGTs. The results show close parallels between the delta(13)CbrGDGT values and OM in the downcore profiles for sediment sequences from two lakes. In addition, the delta(13)CbrGDGT values were consistently more negative than those of OM for the lakes and soil samples from the surrounding watershed. The values are consistent with a mixture of heterotrophic sources and preferential utilization of delta C-13-depleted carbon sources. The delta C-13 values of individual brGDGT components confirm in situ production of brGDGTs within lacustrine dysoxic environments and suggest that their association with an elevated CH4 level during the summer may reflect methanotrophy. (C) 2017 Elsevier Ltd. All rights reserved.