The use of multivariate statistics to resolve multiple contamination signals in the oxygen isotope analysis of biogenic silica

Analysis of the oxygen isotope composition (δ18O) of diatom silica is a commonly used tool for palaeoclimate reconstruction that recent studies have demonstrated may be complicated by the presence of non-diatom detrital material. Such contamination can mask any true climate-driven signal, leading to spurious results. Analysis of the 2.6-Ma Barsemoi diatomites from the East African Rift Valley highlights the presence of both tephra and clay in purified samples. Here we present a new method for assessing the relative contribution and geochemical composition of contamination components where sedimentary samples may be affected by more than one type of contamination. This approach shows that the incorporation of analytical techniques, such as X-ray fluorescence spectrometry, coupled with statistical modelling, can be used to develop a three end-member model to successfully resolve climate-driven changes in δ18Odiatom. Mass-balance corrections made to δ18Odiatom data demonstrate the importance of adopting quantitative geochemical analysis in tandem with the δ18O analysis of biogenic silica, to obtain accurate and meaningful results for palaeoclimate reconstruction

Palaeoclimate Reconstruction in the Levant and on the Balkans

For an understanding of the climate system’s variability, knowledge of the past climate is essential. Continuous observations are not available for longer than the last 100 years, a period that is insufficient to understand the variability and sensitivity of the climate system. Since information of both is necessary to build good climate models, the reconstruction of the past climate is fundamental. With palaeoclimate reconstructions it is possible to get information about the past climate and the climate changes in the period and region of interest. In this work these are the Levant region and the Balkans. The Levant region is situated around the Jordan Valley in Israel. In this work the presence of pollen and macrofossils is used as a proxy. In detail the method bases on the assumption that the presence of a plant or more general of biometypes in a certain area is addicted to the climate. This connection between the occurrence of the plants and the climate is described by transfer functions. The nature of these transfer functions has to be probabilistic because the climate-biosphere system is a stochastical system. In the Collaborative Research Centre (CRC) 806 project B3 ”Our way to Europe” high-resolution lacustrine sediment cores were drilled in March 2010 by Thomas Litt and his working group at Lake Kinneret and Birkat-Ram. The sediment core situated on the Balkans was retrieved at Lake Prespa in November 2009 as part of the CRC project B2. This study presents the results of local palaeoclimate reconstructions based on methods which are a statistical extension of the concept of biomisation, plant functional types and mutual climatic range (MCR). In more detail the Bayesian Biome Model (BBM) is applied for Lake Kinneret and Ein Gedi and the Bayesian Indicator Taxa Model (BITM) for Lake Prespa. For Birkat Ram the Bayesian Indicator Taxa and Biome Model (BITBM) is newly developed and applied. This method combines the BBM and BITM. Reconstructed are the near surface temperatures, middle troposphere temperatures (850hPa level), the annual climatic water deficit CWDANN and the annual precipitation amount PANN. All presented palaeoclimate temperature reconstructions except Lake Kinneret share that the surface and the middle troposphere temperature reconstructions are in accordance. It is also shown that theCWDANN palaeoclimate reconstruction works and does not contradict PANN. The marginal distribution forCWDANN is for example for Lake Prespa a reconstructed palaeoclimate variable which allows more identifiable variation than in PANN. For Lake Prespa there are four, for Birkat Ram three, for Ein Gedi also three and for Lake Kinneret no identifiable time ranges with different climate in the marginal probability density function (pdf)s. In the case that there are time ranges with different climate they are clearly identifiable in the palaeoclimate reconstructions since the marginal distribution profiles before and after differ more or less. Some of these time ranges are compared by application of a Student’s t-test for a significance test. Also presented is an interpolation of local reconstructions situated in the Levant or more precise the Jordan Valley which allows a better assessment of climate changes. The Jordan Valley climate field reconstruction (CFR) results is a dryer palaeoclimate than the modern climate for PANN and no climate change for the considered temperatures for all fossil sites and considered time slices. This result remains uncertain since there are some difficulties with the climate database

HOLOCENE PALAEOCLIMATE RECONSTRUCTION FROM N EW ZEALAND PEATLANDS

A selection of palaeoecological proxies were tested on two raised, ombrotrophic, restiad peat bog sites from the North Island of New Zealand. With varying degrees of success, peat humification, testate amoebae, plant macrofossil and charcoal analyses contributed to determining the palaeomoisture records from three cores on each site. Climatic inferences have successfully been made for the Waikato region, mid North Island, over the period between the Tuhua (6,130±30 14C years BP; 6,800-7,150 cal. years BP) and Taupo (1,850±10 14C years BP; 1,650-1,800 cal. years BP) tephras. 47 AMS radiocarbon ages contributed to the production of age-depth models. The peat humification records have a resolution of 20-30 years and are the first replicated records for this region. Whilst peat humification and charcoal were the most successful analyses, the limited plant macrofossil work has also shown potential. Testate amoebae analysis, however, was not appropriate for these particular sites due to extremely low fossil test abundances. From the peat humification record, the main palaeomoisture trend identified in all cores is a shift towards wetter conditions c.5,000-4,000 cal. years BP. This is thought to have resulted from stronger westerly circulation, driven by increasing temperature and pressure gradients across the Southern Ocean from equatorial to polar latitudes. This in turn is likely to have been due to an increasing differential between insolation received at these latitudes, ultimately forced by the precessional cycle. The 'Mid-Holocene Transition' to wet conditions appears to contradict previous work from New Zealand that infers a drier late Holocene. This apparent contradiction can, however, be reconciled by increasing seasonality that would also explain the rise in charcoal abundance following the Mid-Holocene Transition. Colder, wetter winters resulted from decreasing winter (June) insolation and stronger rain-bearing westerlies, causing a decrease in peat humification. Warmer, drier summers resulted from an increase in summer (December) insolation and caused increased regional burning. An intensifying ENSO signal at the mid-Holocene is also thought to be responsible for increased drought occurrence and weather extremes

Drivers of precipitation stable oxygen isotope variability in an alpine setting, Snowy Mountains, Australia

Natural archives that preserve a stable isotopic signature are routinely used to reconstruct palaeoenvironmental conditions. Isotopic values of precipitation are known to be influenced by factors such as the amount and type of precipitation, moisture pathway, landscape and terrain factors, and processes associated with precipitation formation and deposition. This study investigates oxygen isotopic variability using real-time rain and snow precipitation data from a moderate altitude

Multi-indicator conductivity transfer functions for Quaternary palaeoclimate reconstruction

Diatoms (Bacillariophyceae; single-celled algae) and ostracods (Ostracoda; shelled microcrustacea) are known for their sensitivity to salinity. In palaeolimnology, the potential has yet to be tested for quantifying past salinity, lake level, and by inference, climate change, by application of multiple-indicator transfer functions. We used weighted averaging techniques to derive diatom (n=91; r 2 =0.92) and ostracod (n=53; r 2 =0.83) conductivity transfer functions from modern diatom, ostracod and water chemistry data collected in lakes of central, western and northern Turkey. Diatoms were better represented across the full gradient than ostracods, at intermediate levels of conductivity in particular, but both transfer functions were statistically robust. Because transfer functions are not infallible, we further tested the strength and simplicity of salinity response and the potential for identifying characteristic associations of diatom and ostracod taxa in different parts of the salinity gradient, to improve palaeoclimate reconstruction. We identified a subset of 51 samples that contained both diatoms and ostracods, collected at the same time from the same habitat. We used Two-Way Indicator Species Analysis of a combined diatom-ostracod data set, transformed to achieve numerical equivalence, to explore distributions in more detail. A clear ecological threshold was apparent at ~3gl-1 salinity, rather than at 5gl-1, the boundary used by some workers, equating to the oligosaline-mesosaline boundary. Other salinity boundaries were poorly defined, indicating lack of a simple, well-defined salinity response. We did, however, define characteristic associations of taxa, to facilitate the distinction to be drawn between a hydrologically open, fresh lake and an oligosaline lake, in palaeoenvironmental reconstruction. Over the rest of the salinity gradient, we highlighted the potential for the multi-proxy approach to strengthen ostracod-based reconstruction in particular, to overcome the problem of broad apparent tolerance ranges in common halophilic taxa such as Limnocythere inopinata, which often dominate in low-diversity fossil assemblages. The combination of multi-proxy quantitative reconstruction, complemented by qualitative understanding of ecological responses generated by the analysis, remains a powerful tool in Quaternary palaeoclimate research

On Bayesian Modelling of the Uncertainties in Palaeoclimate Reconstruction

We outline a model and algorithm to perform inference on the palaeoclimate and palaeoclimate volatility from pollen proxy data. We use a novel multivariate non-linear non-Gaussian state space model consisting of an observation equation linking climate to proxy data and an evolution equation driving climate change over time. The link from climate to proxy data is defined by a pre-calibrated forward model, as developed in Salter-Townshend and Haslett (2012) and Sweeney (2012). Climatic change is represented by a temporally-uncertain Normal-Inverse Gaussian Levy process, being able to capture large jumps in multivariate climate whilst remaining temporally consistent. The pre-calibrated nature of the forward model allows us to cut feedback between the observation and evolution equations and thus integrate out the state variable entirely whilst making minimal simplifying assumptions. A key part of this approach is the creation of mixtures of marginal data posteriors representing the information obtained about climate from each individual time point. Our approach allows for an extremely efficient MCMC algorithm, which we demonstrate with a pollen core from Sluggan Bog, County Antrim, Northern Ireland.Comment: 25 pages, 7 figure

Uncertainty quantification in palaeoclimate reconstruction

Studying the dynamics of the palaeoclimate is a challenging problem. Part of the challenge lies in the fact that our understanding must be based on only a single realisation of the climate system. With only one climate history, it is essential that palaeoclimate data are used to their full extent, and that uncertainties arising from both data and modelling are well characterised. This is the motivation behind this thesis, which explores approaches for uncertainty quantification in problems related to palaeoclimate reconstruction. We focus on uncertainty quantification problems for the glacial-interglacial cycle, namely parameter estimation, model comparison, and age estimation of palaeoclimate observations. We develop principled data assimilation schemes that allow us to assimilate palaeoclimate data into phenomenological models of the glacial-interglacial cycle. The statistical and modelling approaches we take in this thesis means that this amounts to the task of performing Bayesian inference for multivariate stochastic differential equations that are only partially observed. One contribution of this thesis is the synthesis of recent methodological advances in approximate Bayesian computation and particle filter methods. We provide an up-to-date overview that relates the different approaches and provides new insights into their performance. Through simulation studies we compare these approaches using a common benchmark, and in doing so we highlight the relative strengths and weaknesses of each method. There are two main scientific contributions in this thesis. The first is that by using inference methods to jointly perform parameter estimation and model comparison, we demonstrate that the current two-stage practice of first estimating observation times, and then treating them as fixed for subsequent analysis, leads to conclusions that are not robust to the methods used for estimating the observation times. The second main contribution is the development of a novel age model based on a linear sediment accumulation model. By extending the target of the particle filter we are able to jointly perform parameter estimation, model comparison, and observation age estimation. In doing so, we are able to perform palaeoclimate reconstruction using sediment core data that takes age uncertainty in the data into account, thus solving the problem of dating uncertainty highlighted above

Western Mediterranean climate and environment since Marine Isotope Stage 3: a 50,000-year record from Lake Banyoles, Spain

We present new stable isotope (δ¹⁸Ocalcite and δ¹³ Ccalcite) and diatom data from a 67-m sediment core (BAN II) from Lake Banyoles, northeastern Spain. We reassessed the chronology of the sequence by correlating stable isotope data with a shorter U-series-dated record from the lake, confirming a sedimentological offset between the two cores and demonstrating that BAN II spans Marine Isotope Stages (MIS) 3–1. Through comparison with previous records, the multi-proxy data are used to improve understanding of palaeolimnological dynamics and, by inference, western Mediterranean climate and environmental change during the past ca. 50,000 years. Three main zones, defined by isotope and diatom data, correspond to the MIS. The basal zone (MIS 3) is characterised by fluctuating δ¹⁸Ocalcite and benthic diatom abundance, indicating a high degree of environmental and climate variability, concomitant with large lake-level changes. During the full glacial (MIS 2), relatively constant δ¹⁸Ocalcite and a poorly preserved planktonic-dominated diatom assemblage suggest stability, and intermittently, unusually high lake level. In MIS 1, δ¹⁸Ocalcite and δ¹³Ccalcite initially transition to lower values, recording a pattern of Late Glacial to Holocene change that is similar to other Mediterranean records. This study suggests that Lake Banyoles responds limnologically to changes in the North Atlantic ocean–atmosphere system and provides an important dataset from the Iberian Peninsula, a region in need of longer-term records that can be used to correlate between marine and terrestrial archives, and between the western and eastern Mediterranean