6-6 DeepCHALLA: the consequences of 'uniqueness'

Session: Integrating 20 Years of Scientific Drilling in the East African-Syrian Rift: A Session In Honor of Andrew Cohen, Part I

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All lakes and other locations selected for ICDP-sponsored continental drilling are deemed uniquely suited to address a particular research question, and billed to have unique potential for driving progress in scientific understanding. Lake Chala/Challa is no exception: following discovery that it contained a ~250,000-year lacustrine sequence with near-certain continuous deposition and at least partly varved sediments, this small volcanic crater lake in eastern equatorial Africa was hailed as uniquely suited to fill an important knowledge gap in Quaternary climate dynamics by potentially producing a high-resolution and well-dated equatorial counterpart to the polar climate-proxy records preserved in Greenland and Antarctic ice, and the sub-tropical climate-proxy records preserved in Chinese and Brazilian speleothems. But this high expectation dictated analytical approaches maintaining mm-scale precision throughout 229.149 m of mud (composite post-recovery length; in situ total depth is 214.8 m) when stitching together a master sequence from 210 overlapping core sections; excising all 1262 turbidites >5 mm thick to avoid proxy ‘contamination’ and achieve a best-possible event-free depth scale for age modelling; or matching XRF elemental profiles with line-scan digital images for targeted cryptotephra searches. Deepwater sedimentation throughout lake history has proved a tremendous advantage (ensuring long-term constancy in proxy-climate links) but not always (“continuously varved sediments are proof that nothing interesting ever happened here”). Besides, counting >100,000 perfectly preserved varves is still a challenge when the local year consists of two wet and two dry seasons with fluctuating prominence; and lack of a decent catchment, while ideal for studies of atmospheric inputs, starved Chala's sedimentary archive of common magnetic and mineral proxies. Climate-proxy interpretation being site-specific has stimulated the DeepCHALLA team to allocate major effort on studies of the modern system for proxy calibration and validation, from 12 years of monthly water-column monitoring and settling-particle trapping, to source attribution of proxy carriers along elevational transects from the lakeshore to high up on neighbouring Mt Kilimanjaro. This effort is now beginning to pay off, in the form of robust climate reconstructions along a continuum of time scales, partly (no surprise!) using proxies ‘uniquely’ developed in and for Lake Chala.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10336

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