Complex evolution of Holocene hydroclimate, fire and vegetation revealed by molecular, minerogenic and biogenic proxies, Marais Geluk wetland, eastern Free State, South Africa

TY - JOUR

T1 - Complex evolution of Holocene hydroclimate, fire and vegetation revealed by molecular, minerogenic and biogenic proxies, Marais Geluk wetland, eastern Free State, South Africa

AU - Sjöström, J. K.

AU - Martínez Cortizas, A.

AU - Nylund, A.

AU - Hardman, A.

AU - Kaal, J.

AU - Smittenberg, R. H.

AU - Risberg, J.

AU - Schillereff, D.

AU - Norström, E.

N1 - Funding Information: Mr Gideon Wessels, the landowner of the Marais Geluk farm and wetland, is thanked for guiding us to the wetland, the sampling permission and for participating in the field-work. Prof Rioual and two reviewers are thanked for thoughtful and valuable comments that helped to improve the manuscript. We are also grateful to Associate Professor Frank Neumann (North-West University, SA) and Dr Jemma Finch (University of KwaZulu-Natal, SA) for data and interpretation discussions. Dr Malin Kylander (Stockholm University, Sweden) is thanked for conducting the CS-XRF analysis, and providing interpretation guidance. Madalina Jaggi (ETH Zurich, Switzerland) is thanked for assistance with EA-IRMS analysis and Noelia River Chaver is thanked for conducting the FTIR-ATR (Universidad de Santiago de Compostela, Spain). E. Norström, J. Sjöström and A. Nylund were funded by the Swedish Research council (project no: 2019–05054). A. Hardman was funded by the European Molecular Biology Organization (grant no: 9703). The FTIR-ATR and pyrolysis-GC-MS analysis were funded by Grupos de Competitiva (ED431C 2021/32), Xunta de Galicia. Funding Information: Mr Gideon Wessels, the landowner of the Marais Geluk farm and wetland, is thanked for guiding us to the wetland, the sampling permission and for participating in the field-work. Prof Rioual and two reviewers are thanked for thoughtful and valuable comments that helped to improve the manuscript. We are also grateful to Associate Professor Frank Neumann (North-West University, SA) and Dr Jemma Finch (University of KwaZulu-Natal, SA) for data and interpretation discussions. Dr Malin Kylander (Stockholm University, Sweden) is thanked for conducting the CS-XRF analysis, and providing interpretation guidance. Madalina Jaggi (ETH Zurich, Switzerland) is thanked for assistance with EA-IRMS analysis and Noelia River Chaver is thanked for conducting the FTIR-ATR (Universidad de Santiago de Compostela, Spain). E. Norström, J. Sjöström and A. Nylund were funded by the Swedish Research council (project no: 2019–05054). A. Hardman was funded by the European Molecular Biology Organization (grant no: 9703 ). The FTIR-ATR and pyrolysis-GC-MS analysis were funded by Grupos de Competitiva (ED431C 2021/32), Xunta de Galicia . Publisher Copyright: © 2023 The Authors

PY - 2023/8/15

Y1 - 2023/8/15

N2 - The Holocene climate history of Southern Africa remains inconclusive despite the increasing number of proxy records from the region. This might be related to the diversity of proxy records, how the proxies are interpreted, or that proxies may respond to more than one forcing (e.g. hydroclimate, fire, temperature.). Here, a 175-cm peat sequence from Free State, South Africa (28°17′53″S, 29°25′10.9″E), was analyzed using a comprehensive set of novel and conventional proxies, including isotopic (δ13C), elemental (CS-XRF), mineral (pXRD), molecular (FTIR-ATR and pyrolysis-GC-MS), grain size (Malvern 3000) and GSSC phytolith composition. The chronology was constructed through AMS radiocarbon dating (n = 7). The early Holocene (10,380–7000 cal yr BP) was characterized by an initial wet phase, followed by relative dryness, at least seasonally, evidenced by slow accumulation rates, low organic content and dominance of terrestrial vegetation in the organic matter matrix. From 7000 cal yr BP, decreasing temperatures, as evidenced in regional climate reconstructions, were associated at Marias Geluk with higher biogenic silica and organic matter content and an increase of moisture-adapted grasses, indicating increasingly mesic conditions. This trend was amplified after 6000 cal yr BP, co-occurring with a southward displacement of the ITCZ. Complex proxy dynamics were observed between 4300 and 2180 cal yr BP, with bulk organic proxies indicating a drier environment (lower carbon content, slow accumulation rates, enriched δ13C values) but the phytolith record pointing towards relatively mesic conditions. The period was also associated with increased fire frequency, that also reached the local wetland. We suggest that the period was associated with seasonally mesic conditions together with increased fire incidence, which affected some of the organic proxies. Increased fire activity was also recorded in the region, while the hydroclimatic indications differed. The last 2000 years, during which human activity is known to have increased in the region, was characterized by lower fire incidence and variable, but relatively moist, conditions. The hydroclimatic inferences for the last 2000 years are in line with previous studies from the region, but additional studies are needed to decipher if the decline in fire incidence was associated to climate forcings, human activities, or a combination of both. The multiproxy approach applied here - in particular the inclusion of FTIR-ATR and pyrolysis GC-MS - revealed a complex interplay between vegetation dynamics, hydrology and paleofire variability. This study confirms that relatively small Holocene temperature variations (compared to northern higher latitudes) were associated with major hydrological variability at Marais Geluk, and reinforces concerns from earlier studies that the hydroclimate of the region is vulnerable to climate change. The result s also show that the southward displacement of the ITCZ, and associated tropical air masses, likely had significant effects on regional hydrology and fire incidence.

AB - The Holocene climate history of Southern Africa remains inconclusive despite the increasing number of proxy records from the region. This might be related to the diversity of proxy records, how the proxies are interpreted, or that proxies may respond to more than one forcing (e.g. hydroclimate, fire, temperature.). Here, a 175-cm peat sequence from Free State, South Africa (28°17′53″S, 29°25′10.9″E), was analyzed using a comprehensive set of novel and conventional proxies, including isotopic (δ13C), elemental (CS-XRF), mineral (pXRD), molecular (FTIR-ATR and pyrolysis-GC-MS), grain size (Malvern 3000) and GSSC phytolith composition. The chronology was constructed through AMS radiocarbon dating (n = 7). The early Holocene (10,380–7000 cal yr BP) was characterized by an initial wet phase, followed by relative dryness, at least seasonally, evidenced by slow accumulation rates, low organic content and dominance of terrestrial vegetation in the organic matter matrix. From 7000 cal yr BP, decreasing temperatures, as evidenced in regional climate reconstructions, were associated at Marias Geluk with higher biogenic silica and organic matter content and an increase of moisture-adapted grasses, indicating increasingly mesic conditions. This trend was amplified after 6000 cal yr BP, co-occurring with a southward displacement of the ITCZ. Complex proxy dynamics were observed between 4300 and 2180 cal yr BP, with bulk organic proxies indicating a drier environment (lower carbon content, slow accumulation rates, enriched δ13C values) but the phytolith record pointing towards relatively mesic conditions. The period was also associated with increased fire frequency, that also reached the local wetland. We suggest that the period was associated with seasonally mesic conditions together with increased fire incidence, which affected some of the organic proxies. Increased fire activity was also recorded in the region, while the hydroclimatic indications differed. The last 2000 years, during which human activity is known to have increased in the region, was characterized by lower fire incidence and variable, but relatively moist, conditions. The hydroclimatic inferences for the last 2000 years are in line with previous studies from the region, but additional studies are needed to decipher if the decline in fire incidence was associated to climate forcings, human activities, or a combination of both. The multiproxy approach applied here - in particular the inclusion of FTIR-ATR and pyrolysis GC-MS - revealed a complex interplay between vegetation dynamics, hydrology and paleofire variability. This study confirms that relatively small Holocene temperature variations (compared to northern higher latitudes) were associated with major hydrological variability at Marais Geluk, and reinforces concerns from earlier studies that the hydroclimate of the region is vulnerable to climate change. The result s also show that the southward displacement of the ITCZ, and associated tropical air masses, likely had significant effects on regional hydrology and fire incidence.

KW - CS-XRF

KW - FTIR

KW - Geochemistry

KW - Holocene

KW - Hydroclimate

KW - Multiproxy

KW - Paleofire

KW - Phytoliths

KW - Pyrolysis-GC-MS

KW - South Africa

UR - http://www.scopus.com/inward/record.url?scp=85165180068&partnerID=8YFLogxK

U2 - 10.1016/j.quascirev.2023.108216

DO - 10.1016/j.quascirev.2023.108216

M3 - Article

AN - SCOPUS:85165180068

SN - 0277-3791

VL - 314

JO - QUATERNARY SCIENCE REVIEWS

JF - QUATERNARY SCIENCE REVIEWS

M1 - 108216

ER -