Extended Data Fig. 5: Coherence between the BIT-index record and seismic stratigraphy of past lake-depth fluctuation in Lake Chala, two independent proxies of past variation in climatic moisture balance.

a, BIT-index record of the last 2150 years at decade-scale resolution (Challacea data¹²⁹), with three episodes of relative drought (majority of BIT-index values <0.65) highlighted in yellow. These periods correlate to lenses L0, L1 and L1bis of basin-focused (i.e., lake low-stand) sedimentation in the uppermost ~2 m (122–125 milliseconds two-way travel time of the seismic-reflection signal, ms TWT) of seismic-reflection stratigraphy at the DeepCHALLA drill site (orange highlights in c; modified from ref. ¹⁴). The three short-lived BIT-index minima highlighted in pink are coeval with three historically documented episodes of widespread regional drought within the last 200 years¹³⁰. b, BIT-index records of the last 20 kyr at century-scale resolution from the Challacea (green³⁴) and DeepCHALLA (orange; this study) sites, with prolonged episodes of relative drought highlighted in yellow and correlated to lenses L0-L5 and Unit 3 (containing L6) of basin-focused sedimentation in the uppermost ~23 m (122–146 ms TWT) of seismic-reflection stratigraphy (c). Note that the TWT scale is not a truly linear depth scale, as it is influenced by the relative strength of successive reflectors. The excellent match between the two 20-kyr BIT-index records from drill sites ~650 m apart (Extended Data Fig. 1) demonstrates i) successful transfer of ²¹⁰Pb/¹⁴C-based chronology from the Challacea sequence⁶⁹ to the equivalent section of the DeepCHALLA sequence through visual cross-correlation of finely laminated sediments shared by both; and ii) considering that the DeepCHALLA site is closer to shore than the Challacea site, and can thus be expected to receive a greater influx of GDGTs produced in catchment soils, the near-identical BIT-index values at both sites indicate that all brGDGTs used in the calculation of the Chala BIT index must result from microbial production within the water column of Lake Chala²¹.