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Author ORCID Identifier
Campus-Only Access for Five (5) Years
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Raymond S. Bradley
Isla S. Castañeda
Biogeochemistry | Climate | Sedimentology
Climate change is an urgent, complicated, and challenging issue for human society today. Changing climate may have large and unexpected influences on the global environment, leading to severer living conditions for billions of people on our planet. To better elucidate relationships between climate change and human societies, we can look back and try to disentangle climate–human–environment interactions in the past. Because the period of instrumental measurements is quite short, it is important to reconstruct past climate using different proxies in geologic materials (Bradley, 2014). Among different proxies in a variety of natural archives, organic biomarkers preserved in lake sediments stand out for providing continuous, high-resolution, and comprehensive climate information from massive locations across the globe (Castañeda and Schouten, 2011).
Organic molecular biomarkers in lake sediments are the individual compounds that can be used to trace to the particular source organism, bioprocess, and the environmental conditions at the time of deposition (Castañeda and Schouten, 2011). Among a wealth of biomarkers, branched glycerol dialkyl glycerol tetraethers (brGDGTs) and leaf wax (n-alkanes) are considered to be useful tools, and have been widely applied to lake sediments to generate temperature and hydroclimate reconstructions (e.g., Thomas et al., 2018). BrGDGTs are bacterial membrane lipids, containing tetra- (I), penta- (II), or hexamethylated (III) components, with zero (a), one (b), or two (c) cyclopentyl moieties (Sinninghe Damsté et al., 2000; Weijers et al., 2006; De Jonge et al., 2013). These compounds are ubiquitous in peats, soils, lacustrine and marine sediments (Schouten et al., 2013). Although the brGDGTs source organisms are still largely unknown (Sinninghe Damsté et al., 2018; van Bree et al., 2020), the methylation index of brGDGTs (MBT′5ME) of surface soils and peat is significantly correlated with mean annual air temperature (MAAT) on a global scale (Weijers et al., 2007; De Jonge et al., 2014a, 2019; Naafs et al., 2017a, 2017b; Dearing Crampton-Flood et al., 2020). Therefore, brGDGTs in natural archives are promising paleothermometers to reconstruct past temperatures. The n-alkanes form the waxy surface of plant leaves. The mid-chain n-alkanes (C21, C23, and C25) are considered as the production of submerged and emergent aquatic plants (Ficken et al., 2000; Hou et al., 2006; Nichols et al., 2009; Gao et al., 2011) while the long-chain length n-alkanes (C25 and higher) are mainly synthesized by terrestrial plants as a coating to protect from water loss (Eglinton and Hamilton, 1967). For the mid-chain n-alkanes, the hydrogen isotopes of mid-chain n-alkanes reflect lake water δ2H with a biosynthetic fractionation (Huang et al., 2004; Sachse et al., 2004; Aichner et al., 2017; Rach et al., 2017; Thomas et al., 2018). Additionally, the hydrogen isotopes of long-chain n-alkanes have been widely used in temperate and the Arctic regions as a proxy for soil water and/or summer precipitation isotopic compositions, plus leaf water evaporative enrichment (Shuman et al., 2006; Aichner et al., 2010; Thomas et al., 2016, 2018; Rach et al., 2017; O’Connor et al., 2020).
One example of climate–human–environment interactions is the mystery of the abandonment of the Norse Settlements. It is actively debated that whether climate change was accountable for the sudden demise of their settlements from southern Greenland in the early 15th century. Erik the Red led the Norse people (“Vikings”) to develop the Eastern Settlement in southern Greenland in 985 CE (Jones, 1986; McGovern, 2014). however, by the early 15th century, Eastern Settlement was abandoned, and the population dropped sharply to near zero (Ledger et al., 2014). It is likely that a changing climate would have led to the failure of agriculture, then the collapse of the Norse Settlement (Berglund, 1986). Unfortunately, most paleotemperature reconstructions from southern Greenland either do not have sufficient temporal resolution to capture such a transition, are not directly calibrated to temperature, or are located relatively far from the inner settlement region (Andresen et al., 2004; Jensen et al., 2004; Massa et al., 2012b; Millet et al., 2014; Winsor et al., 2014; Larsen et al., 2016; Lasher and Axford, 2019).
BrGDGTs in Lake 578 and Lake Igaliku, southern Greenland, were analyzed to acquire the past temperature reconstruction. This paleothermometer has been used in reconstructing past temperature in the Arctic (e.g., de Wet et al., 2016), however, the widely employed temperature-brGDGTs calibrations were based upon the datasets from tropical and/or mid-latitude lakes (Dang et al., 2018; Russell et al., 2018). Recently, more empirical correlations between temperature and brGDGT distributions are published (Martínez-Sosa et al., 2021; Raberg et al., 2021; Stefanescu et al., 2021; Wang et al., 2021; Zhao et al., 2021), but the brGDGTs dataset in the Arctic (Raberg et al., 2021; Zhao et al., 2021) is still limited. To further validate this paleothermometer in the Arctic region, my colleagues and I carried out 3 field campaigns in summers from 2017 to 2019 to collect modern samples, including sediment core, soils, lake surface sediments, and settling particulate matter (SPM) in the lake water column. Based on continuous 3-year SPM samples from Lake 578, southern Greenland, we provided the first site-specific brGDGT lacustrine water temperature calibration and show that the sedimentary brGDGTs could be employed to quantitatively reconstruct summer water temperature in the past. This site-specific calibration was applied to a well-dated sediment core from the same lake, and we established a decadal resolution summer water temperature record over the past 1700 years. This reconstruction reveals a gradual cooling trend in the Late Holocene, but there is no abrupt temperature decline around the time when Eastern Settlements were abandoned.
If the temperature is not the reason for the abandonment of the Eastern Settlement, how about the changes of hydroclimate? Leaf wax hydrogen isotopes in Lake 578 were used to reveal the past hydrological conditions. We measured the hydrogen isotopes of both mid- (nC23) and long-chain (nC29) n-alkanes, which are sourced predominantly from aquatic and terrestrial higher plants, respectively (Eglinton and Hamilton, 1967; Ficken et al., 2000; Shuman et al., 2006; Aichner et al., 2010; Kahmen et al., 2013; Thomas et al., 2016, 2018; Bush et al., 2017; Daniels et al., 2017; Rach et al., 2017). We specifically targeted aquatic and terrestrial waxes isotopes that are influenced by the same processes, but the evaporative enrichment of terrestrial plant leaf water. This allowed us to investigate summer relative humidity (ΔRH) (Kahmen et al., 2013; Rach et al., 2017; Thomas et al., 2018), and we established the first ΔRH records from southern Greenland. This ΔRH reconstruction suggests that this region experienced a persistent drying trend during the settlement period, which peaked in the 16th century. The trend towards drier conditions is concurrent with a shift in the Norse diet from one based on terrestrial animals to one that relied on sea mammals. A drier environment would have significantly reduced grass production, which was essential for livestock over-wintering. Therefore, we conclude that increasingly dry conditions played a more important role in undermining the viability of the Eastern Settlement than minor temperature changes.
We also provide the first monthly resolution brGDGTs records from Lake Igaliku, southern Greenland. The investigation of monthly resolution SPM provides insight information of the spatial variations of in situ brGDGTs. The production of in situ brGDGTs shows strong seasonality and the peak flux is found in October 2018, when Lake Igaliku experienced the isothermal mixing. The seasonal production may alter the interpretation of brGDGTs preserved in lake sediments. This caveat, plus the anomalous “core-top cooling” phenomenon found in Lake 578, shows that we still know little about brGDGTs, and more research is needed to better interpret brGDGT signals in the future.
This work will be of interest to researchers in the fields of climate-human-environment interactions, organic geochemistry, stable isotope geochemistry, paleoclimate, history, and archaeology.
Zhao, Boyang, "ORGANIC BIOMARKER BASED CLIMATE RECONSTRUCTION IN SOUTHERN GREENLAND AND ITS RELATIONSHIP TO THE DEMISE OF NORSE SETTLEMENTS" (2021). Doctoral Dissertations. 2391.
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