Climate change since the Last Interglacial in northern New Zealand inferred from pollen and chironomid records of the Auckland Maars

<p>In light of contemporary climate change it is more important than ever to understand past shifts in climate, especially past warm phases, and their effects on ecosystems and societies. From compilations of global climate reconstructions, several periods have been identified that might have been warmer than today, the two most recent of which are the Holocene Thermal Maximum (~11 – 5 kyr BP) and the Last Interglacial (~129 – 116 kyr BP). However, spatio-temporal complexities are typically smoothed out in global climate reconstructions and we do not have a good understanding of the regional differences in past climate. The southern mid-latitudes especially are underrepresented in palaeoclimate research. For this thesis I analyse the sediments from two maars within the Auckland Volcanic Field: Orakei Basin, which erupted ~126.0 kyr BP and accumulated sediments until ~9 – 8.5 kyr BP; and Lake Pupuke, which still contains a lake today and therefore covers the Holocene. Quantitative climate reconstructions are necessary to put the Orakei Basin and Lake Pupuke records in a broad context and to enable comparisons of past and future climates. For this study I focus on biological proxies preserved by lake sediments, namely pollen, which primarily responds to mean annual air temperatures (MAAT), and chironomids, a surrogate for summer air temperatures (SmT). Together, MAAT and SmT reconstructions from the same site can provide insight into changing seasonality over time, an underexplored dimension of proxy-based reconstructions. The chironomid record covers just the last ~14 cal kyr BP however, because of low head capsule abundances in older sediment sections. The Orakei Basin pollen record and associated MAAT reconstruction cover ~85 to 9 cal kyr BP and show five distinct phases comparable to Marine Isotope Stages (MIS) 5 to 1. This association is confirmed by the preliminary tephrochronology of the core. The broad similarity of the Orakei MAAT trend to the MIS and other records from New Zealand implies all were driven by northern high-latitude summer insolation, consistent with the Milankovitch orbital forcing hypothesis. Several patterns superimposed on the general trend stand out: first, MIS 4 is a brief cool period, which is inconsistent with the observation that glacier advances equivalent to those of the late last glacial maximum occurred ~65 kyr BP in the Southern Alps, possibly due to the seasonal distribution of energy from solar insolation. Second, MIS 3 displays an earlier warm phase followed by a progressive cooling trend which might be correlated to decreasing local summer insolation intensity. Third, glacial conditions of MIS 2 appear consistent with the early onset of the last glacial maximum in the southern mid latitudes, which was likely driven by regional insolation intensity. The Lake Pupuke pollen and chironomid records, covering the last ~14 cal kyr BP, show no evidence of a past warm period equivalent to the Holocene Thermal Maximum. MAAT is stable throughout the Holocene, whereas SmT increases between 10 and 3 cal kyr BP. The latter shows a strong relationship with integrated local summer insolation. The temperature reconstructions lead to the conclusion, first, that seasonality was low during the Early Holocene (12 to 9.3 cal kyr BP), and second, that during mid-to-late Holocene (after ~7 cal kyr BP) summers were hot and dry, allowing the tall conifer kauri to expand throughout northern New Zealand. The Lake Pupuke chironomid-SmT reconstruction highlighted an issue with the transfer function model, namely, that it was not able to reconstruct values close to modern day (18.9°C). Therefore, I explore an extended training set which encompasses a longer temperature gradient. New models are fitted using both traditional techniques and modern machine learning methods. The new model improves the SmT reconstruction from Lake Pupuke, in the sense that reconstructed temperatures now reach modern day values. However, the SmT trend is the same as the original trend, substantiating the previously drawn conclusions. During the course of this research, I discovered that density separation during pollen preparation can lead to varying relative abundances, depending on the specific gravity used. After some experimentation I found that using a low specific gravity (2.0; recommended value in the literature) can result in the overrepresentation of buoyant pollen grains, leading to erroneous interpretations. Together, these results point out the importance of considering regional-to-local drivers of climate changes superimposed on global reconstructions. Multi-proxy records can help disentangle the different aspects of the climate system, where especially chironomids can be helpful to elucidate the role of SmT and local summer insolation. Finally, this thesis shows the importance of questioning the appropriateness of conventional methodologies and where possible, addressing their limitations.</p>

Climate change since the Last Interglacial in northern New Zealand inferred from pollen and chironomid records of the Auckland Maars

<p>In light of contemporary climate change it is more important than ever to understand past shifts in climate, especially past warm phases, and their effects on ecosystems and societies. From compilations of global climate reconstructions, several periods have been identified that might have been warmer than today, the two most recent of which are the Holocene Thermal Maximum (~11 – 5 kyr BP) and the Last Interglacial (~129 – 116 kyr BP). However, spatio-temporal complexities are typically smoothed out in global climate reconstructions and we do not have a good understanding of the regional differences in past climate. The southern mid-latitudes especially are underrepresented in palaeoclimate research. For this thesis I analyse the sediments from two maars within the Auckland Volcanic Field: Orakei Basin, which erupted ~126.0 kyr BP and accumulated sediments until ~9 – 8.5 kyr BP; and Lake Pupuke, which still contains a lake today and therefore covers the Holocene. Quantitative climate reconstructions are necessary to put the Orakei Basin and Lake Pupuke records in a broad context and to enable comparisons of past and future climates. For this study I focus on biological proxies preserved by lake sediments, namely pollen, which primarily responds to mean annual air temperatures (MAAT), and chironomids, a surrogate for summer air temperatures (SmT). Together, MAAT and SmT reconstructions from the same site can provide insight into changing seasonality over time, an underexplored dimension of proxy-based reconstructions. The chironomid record covers just the last ~14 cal kyr BP however, because of low head capsule abundances in older sediment sections. The Orakei Basin pollen record and associated MAAT reconstruction cover ~85 to 9 cal kyr BP and show five distinct phases comparable to Marine Isotope Stages (MIS) 5 to 1. This association is confirmed by the preliminary tephrochronology of the core. The broad similarity of the Orakei MAAT trend to the MIS and other records from New Zealand implies all were driven by northern high-latitude summer insolation, consistent with the Milankovitch orbital forcing hypothesis. Several patterns superimposed on the general trend stand out: first, MIS 4 is a brief cool period, which is inconsistent with the observation that glacier advances equivalent to those of the late last glacial maximum occurred ~65 kyr BP in the Southern Alps, possibly due to the seasonal distribution of energy from solar insolation. Second, MIS 3 displays an earlier warm phase followed by a progressive cooling trend which might be correlated to decreasing local summer insolation intensity. Third, glacial conditions of MIS 2 appear consistent with the early onset of the last glacial maximum in the southern mid latitudes, which was likely driven by regional insolation intensity. The Lake Pupuke pollen and chironomid records, covering the last ~14 cal kyr BP, show no evidence of a past warm period equivalent to the Holocene Thermal Maximum. MAAT is stable throughout the Holocene, whereas SmT increases between 10 and 3 cal kyr BP. The latter shows a strong relationship with integrated local summer insolation. The temperature reconstructions lead to the conclusion, first, that seasonality was low during the Early Holocene (12 to 9.3 cal kyr BP), and second, that during mid-to-late Holocene (after ~7 cal kyr BP) summers were hot and dry, allowing the tall conifer kauri to expand throughout northern New Zealand. The Lake Pupuke chironomid-SmT reconstruction highlighted an issue with the transfer function model, namely, that it was not able to reconstruct values close to modern day (18.9°C). Therefore, I explore an extended training set which encompasses a longer temperature gradient. New models are fitted using both traditional techniques and modern machine learning methods. The new model improves the SmT reconstruction from Lake Pupuke, in the sense that reconstructed temperatures now reach modern day values. However, the SmT trend is the same as the original trend, substantiating the previously drawn conclusions. During the course of this research, I discovered that density separation during pollen preparation can lead to varying relative abundances, depending on the specific gravity used. After some experimentation I found that using a low specific gravity (2.0; recommended value in the literature) can result in the overrepresentation of buoyant pollen grains, leading to erroneous interpretations. Together, these results point out the importance of considering regional-to-local drivers of climate changes superimposed on global reconstructions. Multi-proxy records can help disentangle the different aspects of the climate system, where especially chironomids can be helpful to elucidate the role of SmT and local summer insolation. Finally, this thesis shows the importance of questioning the appropriateness of conventional methodologies and where possible, addressing their limitations.</p>

crestr An R package to perform probabilistic climate reconstructions using fossil proxies

Statistical climate reconstruction techniques are practical tools to study past climate variability from fossil proxy data. In particular, the methods based on probability density functions (PDFs) are powerful at producing robust results from various environments and proxies. However, accessing and curating the necessary calibration data, as well as the complexity of interpreting probabilistic results, often limit their use in palaeoclimatological studies. To address these problems, I present a new R package (crestr) to apply the CREST method (Climate REconstruction SofTware) on diverse palaeoecological datasets. crestr includes a globally curated calibration dataset for six common climate proxies (i.e. plants, beetles, chironomids, rodents, foraminifera, and dinoflagellate cysts) that enables its use in most terrestrial and marine regions. The package can also be used with private data collections instead of, or in combination with, the provided dataset. It also includes a suite of graphical diagnostic tools to represent the data at each step of the reconstruction process and provide insights into the effect of the different modelling assumptions and external factors that underlie a reconstruction. With this R package, the CREST method can now be used in a scriptable environment, thus simplifying its use and integration in existing workflows. It is hoped that crestr will contribute to producing the much-needed quantified records from the many regions where climate reconstructions are currently lacking, despite the existence of suitable fossil records.

Documentary-based climate reconstructions in the Czech Lands 1501–2020 CE and their European context

Annual and seasonal temperature, precipitation and drought index (SPI, SPEI, Z-index, PDSI) series covering the Czech Lands territory (now the Czech Republic) over 520 years (1501–2020 CE) reconstructed from documentary data combined with instrumental observations were analysed herein. The temperature series exhibits a statistically significant increasing trend, rising from ~1890 and particularly from the 1970s; 1991–2020 represents the warmest and driest 30-year period since 1501 CE. While the long-term precipitation total fluctuations (and derived SPI fluctuations) remain relatively stable with annual and decadal variabilities, past temperature increases are the key factor affecting recent increasing dryness in the SPEI, Z-index and PDSI series. The seasonal temperature series represent a broad European area, while the seasonal precipitation series show lower spatial correlations. A statistical attribution analysis conducted utilizing regression and wavelet techniques confirmed the influence of covariates related to volcanic activity (prompting temporary temperature decreases, especially during summer) and the North Atlantic Oscillation (influential in all seasons except summer) in the Czech climate reconstructions. Furthermore, components tied to multidecadal variabilities in the northern Atlantic and northern Pacific were identified in the temperature and precipitation series and in the drought indices, revealing notable shared oscillations, particularly at periods of approximately 70–100 years.

The Last Glacial Maximum and Deglaciation in Southern New Zealand: New Pollen-Climate Reconstructions

<p>The project builds upon existing knowledge of late Quaternary palaeoenvironmenta change and tests the recently developed New Zealand INTIMATE (Integration of Ice Marine and Terrestrial archive) climate event stratigraphy (NZ-I CES; 30-8 ka). Four pollen and sediment records from three climatically contrasting regions in the South Island provide a vegetation and climate history for this area between 38-4 ka. In this study, the Last Glacial Cold Period (LGCP; c. 31.4-18.9 ka) is characterised by a two step cooling, with the coldest conditions, reaching possibly >5.3°C cooling, occurring between 21-19 ka, marking the Last Glacial Maximum. A new precipitation proxy using macrophyte pollen concentrations at an eastern South Island site suggests dominantly dry conditions prevailed during the LGCP except for two periods of wetter climate around 26-24 ka and 21 ka. The dry periods correspond with evidence of glacial advance, colder environments and possibly increased intensity of the southern westerlies. Conversely, the wet periods coincide with reduced glacial activity, milder climates and decreased westerly wind intensity. Deglaciation began between 18.9-18.4 ka followed by rapid climate amelioration culminating with Dacrydium cuppressinum-dominant lowland forest at western sites as early as 11.9 ka, indicative of the start of the Holocene. A disturbance in forest development occurs between 13.4-11.9 ka in one record and may be indicative of a minor cooling within the timeframe of a late glacial climate reversal recognised in the NZI-CES. Overall the project results (timing and pattern of climate change) broadly align with the NZ-I CES. However, there are some disparities, in particular during the LGCP, which this study suggests began at least 3-4 ka earlier than concluded in the NZ-I CES. The NZ-I CES oversimplifies the complexity of the LGCP which contains evidence of significant climate variability that may be important for an understanding of the possible forcing factors on climate change. The chronology derived from the current study supports recent evidence that points towards a younger, refined age of 25.4 ka for the Kawakawa/Oruanui Tephra, a key chronostratigraphic marker for the LGCP. Pollen-climate models and Environmental Lapse Rates were used to quantify changes in mean annual temperatures with sometimes conflicting results. This research reveals some limitations of the current New Zealand pollen-climate transfer function when applied to reconstruction of cold climate periods in particular. These include a lack of limitations with modern analogues and a number of wide-ranging pollen taxa that encompass a broad climate envelope. The current research also highlights the potential of regional climate regimes and spatial differences in vegetation and inferred climate reconstructions. These differences pose a major limitation for a New Zealand-wide composite. While the NZ-I CES provides a valuable framework of climate change during a period of large climate variability, results of this study highlight aspects that need further consideration and revision.</p>

The Last Glacial Maximum and Deglaciation in Southern New Zealand: New Pollen-Climate Reconstructions

<p>The project builds upon existing knowledge of late Quaternary palaeoenvironmenta change and tests the recently developed New Zealand INTIMATE (Integration of Ice Marine and Terrestrial archive) climate event stratigraphy (NZ-I CES; 30-8 ka). Four pollen and sediment records from three climatically contrasting regions in the South Island provide a vegetation and climate history for this area between 38-4 ka. In this study, the Last Glacial Cold Period (LGCP; c. 31.4-18.9 ka) is characterised by a two step cooling, with the coldest conditions, reaching possibly >5.3°C cooling, occurring between 21-19 ka, marking the Last Glacial Maximum. A new precipitation proxy using macrophyte pollen concentrations at an eastern South Island site suggests dominantly dry conditions prevailed during the LGCP except for two periods of wetter climate around 26-24 ka and 21 ka. The dry periods correspond with evidence of glacial advance, colder environments and possibly increased intensity of the southern westerlies. Conversely, the wet periods coincide with reduced glacial activity, milder climates and decreased westerly wind intensity. Deglaciation began between 18.9-18.4 ka followed by rapid climate amelioration culminating with Dacrydium cuppressinum-dominant lowland forest at western sites as early as 11.9 ka, indicative of the start of the Holocene. A disturbance in forest development occurs between 13.4-11.9 ka in one record and may be indicative of a minor cooling within the timeframe of a late glacial climate reversal recognised in the NZI-CES. Overall the project results (timing and pattern of climate change) broadly align with the NZ-I CES. However, there are some disparities, in particular during the LGCP, which this study suggests began at least 3-4 ka earlier than concluded in the NZ-I CES. The NZ-I CES oversimplifies the complexity of the LGCP which contains evidence of significant climate variability that may be important for an understanding of the possible forcing factors on climate change. The chronology derived from the current study supports recent evidence that points towards a younger, refined age of 25.4 ka for the Kawakawa/Oruanui Tephra, a key chronostratigraphic marker for the LGCP. Pollen-climate models and Environmental Lapse Rates were used to quantify changes in mean annual temperatures with sometimes conflicting results. This research reveals some limitations of the current New Zealand pollen-climate transfer function when applied to reconstruction of cold climate periods in particular. These include a lack of limitations with modern analogues and a number of wide-ranging pollen taxa that encompass a broad climate envelope. The current research also highlights the potential of regional climate regimes and spatial differences in vegetation and inferred climate reconstructions. These differences pose a major limitation for a New Zealand-wide composite. While the NZ-I CES provides a valuable framework of climate change during a period of large climate variability, results of this study highlight aspects that need further consideration and revision.</p>

The Potential to Use Variations in Tree-Ring Geometric Center to Estimate Past Wind Speed Change

Tree radial growth is characterized by not only the annual ring-width increment but also shifts in the tree-ring geometric center (TRGC) if subjected to asymmetric external forcing, such as prevailing winds. Previous dendrochronological studies have used the asymmetric growth derived from tree-ring widths to reconstruct wind speed changes. Here we propose a novel method use quantitative TRGC measurements to estimate wind speed. We investigated TRGC shifts in northeast China, where the prevailing westerly winds are strong and persistent. We found that the TRGC showed significant correlations (r = 0.64, p < 0.01) with wind speed in May-September. The higher tree geometry sensitivity to wind speed obtained with the new method compared to previous ones, suggests the possibility of reconstructing historical wind change and variability in prevailing winds using TRGC. In addition, by correcting tree-ring radius according to their TRGC shifts, the basal area increment (BAI) was calculated. Our new BAI estimation provided stronger correlations with climate than both the standard tree-ring width chronology and a traditional BAI estimation. We suggest that future dendrochronological studies should consider TRGC shifts to increase the accuracy in climate reconstructions.

Historical Forest Management Practices Influence Tree-Ring Based Climate Reconstructions

Tree-ring widths (TRW) of historical and archeological wood provide crucial proxies, frequently used for high-resolution multi-millennial paleoclimate reconstructions. Former growing conditions of the utilized trees, however, are largely unknown. Potential influences of historical forest management practices on climatic information, derived from TRW variability need to be considered but have not been assessed so far. Here, we examined the suitability of TRW series from traditionally managed oak forests (Quercus spp.) for climate reconstructions. We compared the climate signal in TRW chronologies of trees originating from high forests and coppice-with-standards (CWS) forests, a silvicultural management practice widely used in Europe for most of the common era. We expected a less distinct climate control in CWS due to management-induced growth patterns, yet an improved climate-growth relationship with TRW data from conventionally managed high forests. CWS tree rings showed considerably weaker correlations with hydroclimatic variables than non-CWS trees. The greatest potential for hydroclimate reconstructions was found for a large dataset containing both CWS and non-CWS trees, randomly collected from lumber yards, resembling the randomness in sources of historical material. Our results imply that growth patterns induced by management interventions can dampen climate signals in TRW chronologies. However, their impact can be minimized in well replicated, randomly sampled regional chronologies.