Vegetation controls on weathering intensity during the last deglacial transition in southeast Africa.

Sarah J Ivory,Geoffrey S Ellis,Andrew S Cohen,Annie Vincens,Michael M Mcglue,Anne-Marie Lézine,Olivier Boucher

doi:10.1371/journal.pone.0112855

Abstract

Tropical climate is rapidly changing, but the effects of these changes on the geosphere are unknown, despite a likelihood of climatically-induced changes on weathering and erosion. The lack of long, continuous paleo-records prevents an examination of terrestrial responses to climate change with sufficient detail to answer questions about how systems behaved in the past and may alter in the future. We use high-resolution records of pollen, clay mineralogy, and particle size from a drill core from Lake Malawi, southeast Africa, to examine atmosphere-biosphere-geosphere interactions during the last deglaciation (∼18–9 ka), a period of dramatic temperature and hydrologic changes. The results demonstrate that climatic controls on Lake Malawi vegetation are critically important to weathering processes and erosion patterns during the deglaciation. At 18 ka, afromontane forests dominated but were progressively replaced by tropical seasonal forest, as summer rainfall increased. Despite indication of decreased rainfall, drought-intolerant forest persisted through the Younger Dryas (YD) resulting from a shorter dry season. Following the YD, an intensified summer monsoon and increased rainfall seasonality were coeval with forest decline and expansion of drought-tolerant miombo woodland. Clay minerals closely track the vegetation record, with high ratios of kaolinite to smectite (K/S) indicating heavy leaching when forest predominates, despite variable rainfall. In the early Holocene, when rainfall and temperature increased (effective moisture remained low), open woodlands expansion resulted in decreased K/S, suggesting a reduction in chemical weathering intensity. Terrigenous sediment mass accumulation rates also increased, suggesting critical linkages among open vegetation and erosion during intervals of enhanced summer rainfall. This study shows a strong, direct influence of vegetation composition on weathering intensity in the tropics. As climate change will likely impact this interplay between the biosphere and geosphere, tropical landscape change could lead to deleterious effects on soil and water quality in regions with little infrastructure for mitigation.

Highlights

Tropical climate is rapidly changing resulting in altered atmospheric and oceanic circulation, as well as changing variability of important climatic modes like the El Nino–South Oscillation [1]
Core MAL05-2A was collected from the northern basin of Lake Malawi (10u1.19S, 34u11.29E; 359 m water depth) during the Lake Malawi Drilling Project (LMDP) in 2005 [29]
In our interpretation of clay mineralogy, we focus on the ratio of kaolinite to smectite (K/S) as a proxy for chemical weathering intensity, which has been used in other similar studies in Africa [38,39]

Summary

Introduction

Tropical climate is rapidly changing resulting in altered atmospheric and oceanic circulation, as well as changing variability of important climatic modes like the El Nino–South Oscillation [1]. Much work is being done within the framework of the IPCC to better understand climate sensitivity and climateinduced changes to the biosphere; integrating the effects on and feedbacks from the geosphere has been largely unexplored [2]. Alterations to the geosphere, an important component of the Earth’s critical zone, could have costly, hazardous implications for water and soil quality, as well as a host of ecosystems services provided by inland waters (e.g., serving as fisheries, rookeries, and zones of stormwater retention). The low resolution of most marine sedimentary records and the strong influence of long time-scale (106–107 yrs) processes such as orogeny do not provide a scalable framework for evaluating future changes in the critical zone. Climate has long been thought to play a strong, direct role, a number of datasets indicate that the role of climate for weathering and erosion may be indirect, mediated by vegetation and soil storage of organic acids [4,5,6,7]

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Conclusion