Seasonality in equatorial climate over the past 25 k.y. revealed by oxygen isotope records from Mount Kilimanjaro

Abstract

Multi-proxy analysis of a well-dated 25,000-year (25 ka) lake-sediment sequence from Lake Challa, on the eastern flank of Mt Kilimanjaro, reveal the climatic controls which govern both the lake’s palaeohydrology and the climate-proxy record contained in the mountain’s receding ice cap. The oxygen-isotope record extracted from diatom silica (d18Odiatom) in Lake Challa sediments captured dry conditions during the last glacial period and a wet late-glacial transition to the Holocene interrupted by Younger Dryas drought. Further, it faithfully traced gradual weakening of the southeastern monsoon during the Holocene. Overall, d18Odiatom matches the branched isoprenoid tetraether (BIT) index of rainfall-induced soil run-off, except during 25–22 ka BP and the last 5 ka when insolation forcing due to orbital precession enhanced the northeastern monsoon. This pattern arises because during these two periods, a weakened southeastern monsoon reduced the amount of rainfall during the long rain season and enhanced the opposing effect of evaporation intensity and/or length of the austral winter dry season. Importantly, our lake-based reconstruction of moisture-balance seasonality in equatorial East Africa also helps understand the oxygen-isotope record contained in Mt. Kilimanjaro ice. Negative correlation between ice-core d18O and Lake Challa d18Odiatom implies that temperature, not moisture balance, is the primary climate control on the long-term trend in ice-core d18O

DeepCHALLA: Two Glacial Cycles of Climate and Ecosystem Dynamics from Equatorial East Africa

“…By contrast, a paleohydrological reconstruction based on the oxygen-isotope (d 18 O) signature of diatom silica, which in this system mainly reflects the duration and severity of the main dry season (Barker et al, 2011), displayed an increasing trend throughout most of the Holocene. Combination of the BIT and diatom d 18 O records showed how decoupled variation in annual rainfall and seasonal drought controlled long-term variation in the region's savanna fire regimes (Nelson et al, 2012).…”

“…Lake-monitoring efforts were stepped up in November 2006 with the installation of air and water temperature loggers and a sediment trap and with the monthly collection of water-level data and water samples for stable-isotope analysis. The monthly sediment-trap samples provided invaluable information on seasonal variation in phytoplankton composition, carbonate precipitation, mineral dust, and organic matter deposition (Barker et al, 2011;Wolff et al, 2011), and on the influx of organic biomarkers derived from a variety of aquatic and terrestrial microorganisms ). All these data and samples are used to calibrate and validate climate and environmental proxies extracted from the sediment record.…”

Cultural drivers of reforestation in tropical forest groves of the Western Ghats of India

“…Furthermore, some soil types such as calcium-rich or alkaline soils, factors such as aridity, and geomorphological contexts such as hill slopes do not provide suitable depositional environments, thus restricting the availability of sediment sequences even further. As such, many influential long-term ecological studies from the tropics have been based on interpretation of sediment sequences from a small number of sites, sometimes even one site (Hodell et al, 2001;Bakker et al, 2008;Barker et al, 2011). Our intact sequences from two tropical forest groves in the Western Ghats afford a unique opportunity to understand the long-term history of these social-ecological systems and to investigate the drivers of vegetation changes in the groves.…”

Origin and palaeoenvironmental significance of C25 and C27 n-alk-1-enes in a 25,000-year lake-sedimentary record from equatorial East Africa

“…Nutrients in the surface waters are mainly replenished by deep water-column mixing in the dry southern hemisphere winter months of June to August, and known to stimulate diatom blooms in Lake Challa (Barker et al 2011;Wolff et al, 2011). Selective growth of the alkene-producing algae during the ensuing "short" rain season may occur because most diatoms are disadvantaged by the developing water-column stratification.…”

“…7E). index is used as a proxy for monsoon rainfall intensity Barker et al, 2011), although the direct mechanistic link remains only partly unresolved (Sinninghe . In conclusion, the relation between the Alkene Index and other hydrological proxies is not unambiguous, but shows interesting features nonetheless.…”