Impact of climate change on the ecology of the Kyambangunguru crater marsh in southwestern Tanzania during the Late Holocene

Abstract

Instrumental records of temperature and hydrological regimes in East Africa evidence frequent droughts with dramatic effects on population and ecosystems. Sources of these climatic variations remain largely unconstrained, partly because of a paucity of Late Holocene records. Here, we present a multi-proxy analysis of a 4-m continuous sediment core collected in the Kyambangunguru crater marsh, in southwest Tanzania, covering the last yrs (cal. BP). We used microscopic (macro-remains, microfossils, palynofacies, pollen), elemental (carbon, nitrogen contents), molecular (br GDGTs, n-alkanes) and compound-specific isotopic (δ 2 H n-alkanes) investigations to reconstruct the environmental history of the marsh. The multi proxy record reveals that, 2500 years ago, the marsh underwent a major ecological transition from a lake to a peatland. Temperature and hydrological reconstructions evidence warmer and drier conditions between 2200 and 860 cal. BP, which probably triggered the establishment of a perennial peatland. This study is one of the first combined temperature and precipitation record of Late Holocene in the region and highlights changes in the spatial distribution of the East African climate regimes. Several cold periods are observed, between 3300 and 2000 cal. BP and since 630 cal. BP, the latter corresponding to the Little Ice Age. Moreover, wetter conditions are reported during the Medieval Climate Anomaly in contrast to other northeastern African records suggesting that Tanzania is located at the transition between two hydro-climatic zones (north-eastern versus southern Africa) and has experienced variable contributions of these two zones over the last millennium.

Development of global temperature and pH calibrations based on bacterial 3-hydroxy fatty acids in soils

“…This global relationship follows a similar trend as the calibration proposed by De Jonge et al (2014) (MAAT = 31.45 × MBT' 5Me − 8.57) and is more robust and accurate than those obtained between the RAN 15 /RAN 17 and MAAT (Table S3). This confirms that the MBT' 5Me index can be applied at a global scale using a simple linear regression model as previously shown (De Jonge et al, 2014;Naafs et al, 2017), in contrast with the RAN 15 and RAN 17 proxies, for which only strong local calibrations with MAAT were found.…”

“…The CBT' index, reflecting the average number of cyclopentyl rings in GDGTs and considered to be related to pH, was calculated as follows (De Jonge et al, 2014; Eq. 5):…”

“…Shennongjia were taken from Yang et al (2015). The global soil calibration by De Jonge et al (2014) was applied to all these transects. Only significant regressions (p < 0.05) are shown.…”

“…Archaeal GDGTs are constituted of isoprenoid alkyl chains ether-linked to glycerol, whereas bacterial GDGTs are characterized by branched alkyl chains instead of isoprenoid ones. The latter compounds are ubiquitous in terrestrial (Weijers et al, 2007;Peterse et al, 2012;De Jonge et al, 2014;Naafs et al, 2017) and aquatic environments (Peterse et al, 2009;Tierney and Russell, 2009;Sinninghe Damsté et al, 2009;Loomis et al, 2012;Peterse et al, 2015;Weber et al, 2015). These branched GDGTs (brGDGTs) are produced by still unidentified bacteria, although some of them may belong to the phylum Acidobacteria (Sinninghe Damsté et al, 2011, 2014.…”

“…These branched GDGTs (brGDGTs) are produced by still unidentified bacteria, although some of them may belong to the phylum Acidobacteria (Sinninghe Damsté et al, 2011, 2014. The analysis of brGDGTs in a large number of soils distributed worldwide showed that the relative distribution of these compounds is mainly related to mean annual air temperature (MAAT) and soil pH (Weijers et al, 2007;Peterse et al, 2012;De Jonge et al, 2014). Even though brGDGT proxies were largely investigated over the last 10 years (De Jonge et al, 2014;Dearing Crampton-Flood et al, 2020) and were applied to various palaeorecords (e.g, Coffinet et al, 2018;Wang et al, 2020), new molecular proxies, independent of and complementary to brGDGTs, are needed to improve the reliability of temperature reconstructions in terrestrial settings.…”