DOI: 10.25921/da6x-9z97
title: NOAA/WDS Paleoclimatology - Lake Challa 25,000 Year BIT Index Data

description: External climate forcings - such as long-term changes in solar insolation - generate different climate responses in tropical and high latitude regions. Documenting the spatial and temporal variability of past climates is therefore critical for understanding how such forcings are translated into regional climate variability. In contrast to the data-rich middle and high latitudes, high-quality climate-proxy records from equatorial regions are relatively few, especially from regions experiencing the bimodal seasonal rainfall distribution associated with twice-annual passage of the Intertropical Convergence Zone. Here we present a continuous and well-resolved climate-proxy record of hydrological variability during the past 25,000 years from equatorial East Africa. Our results, based on complementary evidence from seismic-reflection stratigraphy and organic biomarker molecules in the sediment record of Lake Challa near Mount Kilimanjaro, reveal that monsoon rainfall in this region varied at half-precessional (~11,500-year) intervals in phase with orbitally controlled insolation forcing. The southeasterly and northeasterly monsoons that advect moisture from the western Indian Ocean were strengthened in alternation when the inter-hemispheric insolation gradient was at a maximum; dry conditions prevailed when neither monsoon was intensified and modest local March or September insolation weakened the rain season that followed. On sub-millennial timescales, the temporal pattern of hydrological change on the East African Equator bears clear high-northern-latitude signatures, but on the orbital timescale it mainly responded to low-latitude insolation forcing. Predominance of low-latitude climate processes in this monsoon region can be attributed to the low-latitude position of its continental regions of surface air flow convergence, and its relative isolation from the Atlantic Ocean, where prominent meridional overturning circulation more tightly couples low-latitude climate regimes to high-latitude boundary conditions.
description: The 172 main-series BIT samples (branched and isoprenoid tetraether index of soil bacterial versus aquatic archaeal membrane lipids)  (cf. Fig.2 of the source publication) are 4-cm core increments extracted at constant 12-cm composite-depth intervals, plus 7 additional samples (samples at composite depths 118, 774, 958, 966, 970, 1050, 1074, and 2014 cm) filling in sharp transitions in BIT values (mainly around the Younger Dryas period) and 1 sample replacing the missing sample at 122 cm. The high-resolution BIT time series for the last millennium (cf. Supplementary Fig.7 of the source publication) adds 21 samples (highlighted in blue) from the uppermost 126 cm, to produce a contiguously sampled record of 4-cm core increments. The 3-pt running mean BIT values are based on main-series samples only. 'Composite depth' refers to the continuous depth axis of the master core sequence, which is composed of 21 cross-correlated core sections before excision of 5 turbidites. 'Corrected depth' refers to the depth axis of the master sequence after excision of these turbidites (they occur below samples at composite depths of 482, 674, 1898, 1910, and 2018 cm), and represents a continuous time axis.