A long-term multi-proxy record of varved sediments suggests climate-induced mixing-regime shift in a large hard-water lake ~5000 years ago

The long-term terrestrial and aquatic ecosystem dynamics spanning between approximately 6200 and 4800 cal BP were investigated using pollen, diatoms, pigments, charcoal, and geochemistry from varved sediments collected in a large stratified perialpine lake, Lago Grande di Avigliana, in the Italian Alps. Marked changes were detected in diatom and pigment assemblages and in sediment composition at ~4900 cal BP. Organic matter rapidly increased and diatom assemblages shifted from oligotrophic to oligo-mesotrophic planktonic assemblages suggesting that nutrients increased at that time. Because land cover, erosion, and fire frequency did not change significantly, external nutrient sources were possibly not essential in controlling the lake-ecosystem dynamics. This is also supported by redundancy analysis, which showed that variables explaining significant amounts of variance in the diatom data were not the ones related to changes in the catchment. Instead, the broad coincidence between the phytoplankton dynamics and rising lake-levels, cooler temperatures, and stronger spring winds in the northern Mediterranean borderlands possibly points to the effects of climate change on the nutrient recycling in the lake by means of the control that climate can exert on mixing depth. We hypothesize that the increased P-release rates and higher organic-matter accumulation rates, proceeded by enhanced precipitation of iron sulphides, were possibly caused by deeper and stronger mixing leading to enhanced input of nutrients from the anoxic hypolimnion into the epilimnion. Although we cannot completely rule out the influence of minor land-cover changes due to human activities, it may be hypothesized that climate-induced cumulative effects related to mixing regime and P-recycling from sediments influenced the aquatic-ecosystem dynamics

Lake sedimentary DNA accurately records 20th Century introductions of exotic conifers in Scotland

Summary:Sedimentary DNA (sedDNA) has recently emerged as a new proxy for reconstructing past vegetation, but its taphonomy, source area and representation biases need better assessment. We investigated how sedDNA in recent sediments of two small Scottish lakes reflects a major vegetation change, using well-documented 20th Century plantations of exotic conifers as an experimental system. We used next-generation sequencing to barcode sedDNA retrieved from subrecent lake sediments. For comparison, pollen was analysed from the same samples. The sedDNA record contains 73 taxa (mainly genus or species), all but one of which are present in the study area. Pollen and sedDNA shared 35% of taxa, which partly reflects a difference in source area. More aquatic taxa were recorded in sedDNA, whereas taxa assumed to be of regional rather than local origin were recorded only as pollen. The chronology of the sediments and planting records are well aligned, and sedDNA of exotic conifers appears in high quantities with the establishment of plantations around the lakes. SedDNA recorded other changes in local vegetation that accompanied afforestation. There were no signs of DNA leaching in the sediments or DNA originating from pollen

Human settlement of East Polynesia earlier, incremental, and coincident with prolonged South Pacific drought

The timing of human colonization of East Polynesia, a vast area lying between Hawai‘i, Rapa Nui, and New Zealand, is much debated and the underlying causes of this great migration have been enigmatic. Our study generates evidence for human dispersal into eastern Polynesia from islands to the west from around AD 900 and contemporaneous paleoclimate data from the likely source region. Lake cores from Atiu, Southern Cook Islands (SCIs) register evidence of pig and/or human occupation on a virgin landscape at this time, followed by changes in lake carbon around AD 1000 and significant anthropogenic disturbance from c. AD 1100. The broader paleoclimate context of these early voyages of exploration are derived from the Atiu lake core and complemented by additional lake cores from Samoa (directly west) and Vanuatu (southwest) and published hydroclimate proxies from the Society Islands (northeast) and Kiribati (north). Algal lipid and leaf wax biomarkers allow for comparisons of changing hydroclimate conditions across the region before, during, and after human arrival in the SCIs. The evidence indicates a prolonged drought in the likely western source region for these colonists, lasting c. 200 to 400 y, contemporaneous with the phasing of human dispersal into the Pacific. We propose that drying climate, coupled with documented social pressures and societal developments, instigated initial eastward exploration, resulting in SCI landfall(s) and return voyaging, with colonization a century or two later. This incremental settlement process likely involved the accumulation of critical maritime knowledge over several generations

Palaeoenvironmental investigations of crannogs in south west Scotland and Co. Fermanagh, Northern Ireland

Crannogs are widely distributed archaeological sites in Scotland and Ireland and can be described as artificially constructed islands, dated mainly to the Iron Age and Medieval periods. However, little is known about the function and chronology of these sites. This study aims to show how palaeoecological and palaeoenvironmental analyses can support the interpretation of these sites. Two regions were chosen, as national archaeological databases indicated that they had a high concentration of dated crannogs: south west Scotland and Co. Fermanagh, Northern Ireland. Five lakes in total were selected: Cults Loch, Barhapple Loch and Black Loch of Myrton from south west Scotland, and Derryhowlaght Lough and Ross Lough in Co. Fermanagh. By analysing the lake sediments near the crannogs using palaeoecological/environmental techniques, a more detailed concept of the timing and use of these sites can be formalized. The approach employed in this study explores multiple lines of evidence, where the lake cores were analysed for their geochemical proxies (loss-on-ignition, magnetic susceptibility and X-ray fluorescence) as well as their fossil diatom and pollen assemblages. By examining those sediments dating from around the time of the crannog, disturbances in the lake ecology as a result of the crannog can be identified and explored. The geochemical and diatom records show varying responses, indicating that crannogs had a wide range of impact upon the lake ecology. Some sites indicate eutrophication and acidification, while other sites indicate increased erosion rates. The most common crannog related disturbance is a minor deforestation event around the time of the crannog construction, indicating that the large wooden component of the crannog would have impacted nearby woodlands. In some sites a secondary occupation period showed a stronger palaeoecological response than the estimated depth of the crannog construction. By comparing the long term lake records within these regions, it was also possible to identify large scale regional disturbances. The Scottish crannogs appear to have been built between major deforestation phases in the catchment, while the sites in Co. Fermanagh indicate a major deforestation phase taking place after the main construction phase of the crannogs. In Co. Fermanagh this deforestation phase was synchronous with catchment erosion and increases in planktonic and periphytic diatoms. These disturbances all took place around the end of the Early Medieval Period. There are some limitations to the study, as all of the palaeoecological sequences would benefit from an improved age-depth model and an improved understanding of the interactions between diatom taxa in shallow lakes. Overall this thesis identified the potential of applying palaeoecological analyses to lake cores in these highly disturbed sediments and contributed to an understanding of this common occupation type and place in the landscapes of the North-West of the British Isles

Paleoenvironmental investigations of Crannogs in south west Scotland and Co. Fermanagh, Northern Ireland

Crannogs are widely distributed archaeological sites in Scotland and Ireland and can be described as artificially constructed islands, dated mainly to the Iron Age and Medieval periods. However, little is known about the function and chronology of these sites. This study aims to show how palaeoecological and palaeoenvironmental analyses can support the interpretation of these sites. Two regions were chosen, as national archaeological databases indicated that they had a high concentration of dated crannogs: south west Scotland and Co. Fermanagh, Northern Ireland. Five lakes in total were selected: Cults Loch, Barhapple Loch and Black Loch of Myrton from south west Scotland, and Derryhowlaght Lough and Ross Lough in Co. Fermanagh. By analysing the lake sediments near the crannogs using palaeoecological/environmental techniques, a more detailed concept of the timing and use of these sites can be formalized. The approach employed in this study explores multiple lines of evidence, where the lake cores were analysed for their geochemical proxies (loss-on-ignition, magnetic susceptibility and X-ray fluorescence) as well as their fossil diatom and pollen assemblages. By examining those sediments dating from around the time of the crannog, disturbances in the lake ecology as a result of the crannog can be identified and explored. The geochemical and diatom records show varying responses, indicating that crannogs had a wide range of impact upon the lake ecology. Some sites indicate eutrophication and acidification, while other sites indicate increased erosion rates. The most common crannog related disturbance is a minor deforestation event around the time of the crannog construction, indicating that the large wooden component of the crannog would have impacted nearby woodlands. In some sites a secondary occupation period showed a stronger palaeoecological response than the estimated depth of the crannog construction. By comparing the long term lake records within these regions, it was also possible to identify large scale regional disturbances. The Scottish crannogs appear to have been built between major deforestation phases in the catchment, while the sites in Co. Fermanagh indicate a major deforestation phase taking place after the main construction phase of the crannogs. In Co. Fermanagh this deforestation phase was synchronous with catchment erosion and increases in planktonic and periphytic diatoms. These disturbances all took place around the end of the Early Medieval Period. There are some limitations to the study, as all of the palaeoecological sequences would benefit from an improved age-depth model and an improved understanding of the interactions between diatom taxa in shallow lakes. Overall this thesis identified the potential of applying palaeoecological analyses to lake cores in these highly disturbed sediments and contributed to an understanding of this common occupation type and place in the landscapes of the North-West of the British Isles