Table3_Orbital Influence on Precipitation, Fire, and Grass Community Composition From 1.87 to 1.38 Ma in the Turkana Basin, Kenya.DOCX

The Turkana Basin in northern Kenya and southern Ethiopia has yielded hundreds of hominin fossils and is among the most important localities in the world for studying human origins. High resolution climate and vegetation reconstructions from this region can elucidate potential linkages between hominin evolution and environmental change. Microcharcoal and phytoliths were examined from a 216 m (1.87–1.38 Ma) drill core (WTK13), which targeted paleo-Lake Lorenyang sediments from the Nachukui Formation of the Turkana Basin. A total of 287 samples were analyzed at ∼32–96 cm intervals, providing millennial-scale temporal resolution. To better understand how basin sediments record fire and vegetation from the watershed, the paleorecord was compared with nine modern sediment samples collected from Lake Turkana along a transect of increasing distance from the 1978 to 1979 shoreline. This included vegetation surveys and phytolith production data for species from areas proximal to the basin. We found that phytolith and microcharcoal concentrations decreased predictably moving off shore. However, phytoliths from plants sourced in the Ethiopian Highlands increased moving off shore, likely the result of increased exposure to the Omo River sediment plume. In our down-core study, microcharcoal was well-preserved but phytolith preservation was poor below ∼60 m (∼1.50 Ma). Spectral analysis revealed that microcharcoal often varied at precessional (∼21 kyr) periodicities, and through a correlation with δDwax, linked orbitally forced peaks in precipitation with elevated fire on the landscape. Phytoliths revealed that alternating mesic C4 versus xeric C4 grass dominance likely varied at precessional periodicities as well, but that grass community composition was also mediated by basin geometry. Two high eccentricity intervals of particularly high amplitude and abrupt environmental change were centered at ∼1.72 and 1.50 Ma, with the intervening period experiencing high fire variability. With the switch from lacustrine to fluvial-deltaic deposition at the core site by 1.5 Ma, mesic C4 grasses dominated and fire activity was high. This upper interval correlated to the time interval from which Nariokotome Boy (Homo erectus/ergaster) was discovered 3 km east of our drill site. Phytoliths indicated a seasonally wet and open landscape dominated by xeric C4 grasses, sedges, and other herbaceous plants.

肯尼亚北部和埃塞俄比亚南部的图尔卡纳盆地出土了数百件古人类化石，是研究人类起源的重要地点之一。通过对该地区高分辨率气候和植被的重建，可以阐明古人类进化与环境变化之间的潜在联系。从216米（1.87-1.38百万年前）的钻芯（WTK13）中，对微炭粒和植硅体进行了分析，该钻芯旨在提取图尔卡纳盆地内 Nachukui 地层中古代洛雷扬湖的沉积物。在约32-96厘米的间隔内，总共分析了287个样本，提供了千年时间分辨率的记录。为了更好地理解盆地沉积物如何记录流域的火灾和植被，将古记录与现代沉积样品进行了比较，这些样品是从图尔卡纳湖沿1978至1979年岸线向海方向逐渐增加的距离收集的。这包括了对靠近盆地的物种的植被调查和植硅体生产数据。我们发现，随着离岸距离的增加，植硅体和微炭粒的浓度呈规律性下降。然而，来自埃塞俄比亚高原的植物植硅体在离岸时有所增加，这可能是由于对奥莫河沉积物羽流的暴露增加。在我们的钻芯研究中，微炭粒保存良好，但在约60米（约1.50百万年前）以下，植硅体的保存状况较差。光谱分析显示，微炭粒通常以约21千年的准周期性变化，通过与δDwax的相关性，将降水增加与景观中火灾的加剧联系起来。植硅体揭示了中等湿润的C4与干旱的C4草本植物的交替主导可能也以准周期性变化，但草本群落组成也受到盆地几何形状的调节。在约1.72和1.50百万年前有两个高偏心率间隔，幅度特别大且环境变化突然，其间期经历了高火灾变率。到1.5百万年前，从湖泊到河流-三角洲沉积的转换在核心地点发生，中等湿润的C4草本植物占主导地位，火灾活动频繁。这一上层间隔与在大约3公里远的钻探地点东边发现的纳里奥科托梅男孩（直立人/匠人）的时间段相对应。植硅体表明，以干旱的C4草本植物、莎草和其他草本植物为主的季节性湿润开阔景观。