High-resolution TECHNICAP sediment traps have been deployed in Lake Baikal since 1996. Results of... more High-resolution TECHNICAP sediment traps have been deployed in Lake Baikal since 1996. Results of these experiments show distinct seasonal differences in particle flux, ranging from a few mg m-2d-1 to several g m-2d-1. Analyses of trap material show that removal of particles is very fast and efficient. Downward transport through the 1400 m deep water column reaches velocities of up
ABSTRACT Equatorial East Africa has a complex, regional patchwork of climate regimes, with multip... more ABSTRACT Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000-1200 AD) and a relatively drier climate during the main phase of the LIA (1500-1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response.
ABSTRACT Palaeoenvironmental and 14C reservoir age variability in the Limfjord, a sound through n... more ABSTRACT Palaeoenvironmental and 14C reservoir age variability in the Limfjord, a sound through northern Jutland, Denmark, was investigated for the period 7300 to 1300 cal yr BP. Shells and bulk sediment samples from a core from a former inlet, Kilen, were analysed by radiocarbon dating and stable isotope (C/N) measurements. A strong correlation between the C/N ratios and δ13C values verifies that these are good carbon source indicators and thus allow environmental reconstructions. Furthermore, δ13C values are correlated with salinity in the photic zone, inferred quantitatively from diatom assemblages. They are therefore used to differentiate between brackish and marine palaeo-conditions. 14C reservoir ages of shells vary from Δ R =−140 to +300 14C years. Between 7300 and 5400 cal. yr BP, reservoir age and stable isotope values are highly variable and indicate mixing of marine water and brackish surface waters with hard water effects. After 5400 cal. yr BP, the ΔR values stabilise and show an increasingly marine environment, with 14C reservoir ages close to 400 years (ΔR=0). After 2000 cal. yr BP, Kilen becomes brackish. Reservoir ages and stable isotope values are again highly variable and δ13C and C/N values are no longer correlated. Before 4000 cal. yr BP, δ15N values vary by only 1‰ and do not reflect the changes in the marine environment. A δ15N increase between 3500 and 2000 cal. yr BP signifies enhanced organic productivity or a change in agricultural practices.
ABSTRACT Diatom preservation can be a major taphonomic issue in many lakes but is often unrecogni... more ABSTRACT Diatom preservation can be a major taphonomic issue in many lakes but is often unrecognised and its impacts on qualitative and quantitative inferences (such as productivity and biodiversity estimates) from sedimentary archives are seldom explored. Here two palaeolimnological case studies of 20th-century anthropogenic eutrophication of freshwater lakes in Northern Ireland (Lough Neagh and Lough Augher) are re-visited and new data presented on diatom preservation. Assessing problems of taphonomy challenges previous interpretations of silica dynamics and diatom productivity at these sites. Diatom preservation was assessed in both sediment trap material and sediment cores from Lough Neagh, and in sediment cores from Lough Augher. Preservation data, combined with geochemical analysis (Si, Fe), provide an insight into silica cycling and diatom accumulation over a range of temporal scales from these lakes. Diatom preservation was generally good for the Lough Neagh material, although differential (better) preservation of the smaller Aulacoseira subarctica compared to the larger Stephanodiscus neoastraea sensu lato valves was clear, especially in sediments. Porewater silica showed a complex seasonal pattern in the upper sediment, against expectations of steady-state. The Lough Augher material was generally poorly preserved, although preservation (dissolution) was significantly (and positively) correlated to bulk sedimentation rate, and was found to be a major control on (net) diatom accumulation rate across the basin. Past seasonal and severe anoxia at Lough Augher did not improve diatom preservation, contrary to some previous studies, which may be due to extreme changes in sedimentary redox conditions. Finally, using published experimental relationships between dissolution and diatom valve loss, correction factors were applied to previously published profiles of diatom accumulation over the last ~150 years (biovolume from Lough Neagh and frustule accumulation rate from Lough Augher), which suggest that diatom productivity estimates from sedimentary records are underestimated by a factor of 2–4 due to dissolution effects alone. The results clearly have implications for the reliability and accuracy of diatom-based inferences made from sediment records, both qualitative and quantitative, especially for those that employ diatoms as direct measures of productivity or biodiversity.
ABSTRACT The late Holocene history of the South African summer rainfall zone offers insights into... more ABSTRACT The late Holocene history of the South African summer rainfall zone offers insights into the effects of climate on ecosystems and human societies, as well as into the accuracy of model projections of the future. However, some important aspects of this region’s climatic history remain unresolved. Here we present new high-resolution diatom records representing hydrological fluctuations at Lake Sibaya, KwaZulu-Natal, during the last 1800 years. The cores were dated with 14C, 210Pb, 137Cs, and exotic pollen, and were sampled at increments of 1e22 years. A low stand ending wAD 150 was followed by additional decadal to century-scale droughts, most notably wAD 1540e1760, and several periods of markedly wetter conditions wAD 220e290, AD 790e830, AD 1470e1540, and AD 1760e1860. The Medieval Climate Anomaly was generally wetter than average and the Little Ice Age was generally drier, but hydroclimate during both intervals was highly variable. These records confirm that local tree ring and stalagmite gray scale series represent rainfall variability, but they also show that widely cited stable isotope series from Makapansgat do not represent past climate as clearly. Because many interpretations of the climatic history of southern Africa have been influenced by those isotope data, we re-examine late Holocene precipitation variability in the summer rainfall zone, and also address model projections of future precipitation in the region.
... Two neighbouring (c. 2 km apart) closed-basin saline lakes, Braya Sï (lake SS4; Anderson et a... more ... Two neighbouring (c. 2 km apart) closed-basin saline lakes, Braya Sï (lake SS4; Anderson et al., 2001) and lake SS6, were investigated (Figure 1). They lie in a hilly area (relative relief c. 200 m, maximum altitude c. 450 m) of scrub and heathland (Böcher, 1949) dominated by ...
Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/15/ 1/119 The online ... more Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/15/ 1/119 The online version of this article can be found at: DOI: 10.1191/ 0959683605hl774rp 2005 15: 119 The Holocene Immaculate Ssemmanda ...
High-resolution TECHNICAP sediment traps have been deployed in Lake Baikal since 1996. Results of... more High-resolution TECHNICAP sediment traps have been deployed in Lake Baikal since 1996. Results of these experiments show distinct seasonal differences in particle flux, ranging from a few mg m-2d-1 to several g m-2d-1. Analyses of trap material show that removal of particles is very fast and efficient. Downward transport through the 1400 m deep water column reaches velocities of up
ABSTRACT Equatorial East Africa has a complex, regional patchwork of climate regimes, with multip... more ABSTRACT Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000-1200 AD) and a relatively drier climate during the main phase of the LIA (1500-1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response.
ABSTRACT Palaeoenvironmental and 14C reservoir age variability in the Limfjord, a sound through n... more ABSTRACT Palaeoenvironmental and 14C reservoir age variability in the Limfjord, a sound through northern Jutland, Denmark, was investigated for the period 7300 to 1300 cal yr BP. Shells and bulk sediment samples from a core from a former inlet, Kilen, were analysed by radiocarbon dating and stable isotope (C/N) measurements. A strong correlation between the C/N ratios and δ13C values verifies that these are good carbon source indicators and thus allow environmental reconstructions. Furthermore, δ13C values are correlated with salinity in the photic zone, inferred quantitatively from diatom assemblages. They are therefore used to differentiate between brackish and marine palaeo-conditions. 14C reservoir ages of shells vary from Δ R =−140 to +300 14C years. Between 7300 and 5400 cal. yr BP, reservoir age and stable isotope values are highly variable and indicate mixing of marine water and brackish surface waters with hard water effects. After 5400 cal. yr BP, the ΔR values stabilise and show an increasingly marine environment, with 14C reservoir ages close to 400 years (ΔR=0). After 2000 cal. yr BP, Kilen becomes brackish. Reservoir ages and stable isotope values are again highly variable and δ13C and C/N values are no longer correlated. Before 4000 cal. yr BP, δ15N values vary by only 1‰ and do not reflect the changes in the marine environment. A δ15N increase between 3500 and 2000 cal. yr BP signifies enhanced organic productivity or a change in agricultural practices.
ABSTRACT Diatom preservation can be a major taphonomic issue in many lakes but is often unrecogni... more ABSTRACT Diatom preservation can be a major taphonomic issue in many lakes but is often unrecognised and its impacts on qualitative and quantitative inferences (such as productivity and biodiversity estimates) from sedimentary archives are seldom explored. Here two palaeolimnological case studies of 20th-century anthropogenic eutrophication of freshwater lakes in Northern Ireland (Lough Neagh and Lough Augher) are re-visited and new data presented on diatom preservation. Assessing problems of taphonomy challenges previous interpretations of silica dynamics and diatom productivity at these sites. Diatom preservation was assessed in both sediment trap material and sediment cores from Lough Neagh, and in sediment cores from Lough Augher. Preservation data, combined with geochemical analysis (Si, Fe), provide an insight into silica cycling and diatom accumulation over a range of temporal scales from these lakes. Diatom preservation was generally good for the Lough Neagh material, although differential (better) preservation of the smaller Aulacoseira subarctica compared to the larger Stephanodiscus neoastraea sensu lato valves was clear, especially in sediments. Porewater silica showed a complex seasonal pattern in the upper sediment, against expectations of steady-state. The Lough Augher material was generally poorly preserved, although preservation (dissolution) was significantly (and positively) correlated to bulk sedimentation rate, and was found to be a major control on (net) diatom accumulation rate across the basin. Past seasonal and severe anoxia at Lough Augher did not improve diatom preservation, contrary to some previous studies, which may be due to extreme changes in sedimentary redox conditions. Finally, using published experimental relationships between dissolution and diatom valve loss, correction factors were applied to previously published profiles of diatom accumulation over the last ~150 years (biovolume from Lough Neagh and frustule accumulation rate from Lough Augher), which suggest that diatom productivity estimates from sedimentary records are underestimated by a factor of 2–4 due to dissolution effects alone. The results clearly have implications for the reliability and accuracy of diatom-based inferences made from sediment records, both qualitative and quantitative, especially for those that employ diatoms as direct measures of productivity or biodiversity.
ABSTRACT The late Holocene history of the South African summer rainfall zone offers insights into... more ABSTRACT The late Holocene history of the South African summer rainfall zone offers insights into the effects of climate on ecosystems and human societies, as well as into the accuracy of model projections of the future. However, some important aspects of this region’s climatic history remain unresolved. Here we present new high-resolution diatom records representing hydrological fluctuations at Lake Sibaya, KwaZulu-Natal, during the last 1800 years. The cores were dated with 14C, 210Pb, 137Cs, and exotic pollen, and were sampled at increments of 1e22 years. A low stand ending wAD 150 was followed by additional decadal to century-scale droughts, most notably wAD 1540e1760, and several periods of markedly wetter conditions wAD 220e290, AD 790e830, AD 1470e1540, and AD 1760e1860. The Medieval Climate Anomaly was generally wetter than average and the Little Ice Age was generally drier, but hydroclimate during both intervals was highly variable. These records confirm that local tree ring and stalagmite gray scale series represent rainfall variability, but they also show that widely cited stable isotope series from Makapansgat do not represent past climate as clearly. Because many interpretations of the climatic history of southern Africa have been influenced by those isotope data, we re-examine late Holocene precipitation variability in the summer rainfall zone, and also address model projections of future precipitation in the region.
... Two neighbouring (c. 2 km apart) closed-basin saline lakes, Braya Sï (lake SS4; Anderson et a... more ... Two neighbouring (c. 2 km apart) closed-basin saline lakes, Braya Sï (lake SS4; Anderson et al., 2001) and lake SS6, were investigated (Figure 1). They lie in a hilly area (relative relief c. 200 m, maximum altitude c. 450 m) of scrub and heathland (Böcher, 1949) dominated by ...
Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/15/ 1/119 The online ... more Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/15/ 1/119 The online version of this article can be found at: DOI: 10.1191/ 0959683605hl774rp 2005 15: 119 The Holocene Immaculate Ssemmanda ...
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