Centennial-scale Climate Research Articles

Reconstructing Holocene centennial cooling events: synthesized temperature changes, chronology, and forcing in the Northern Hemisphere

Numerous studies, spanning experimental, instrumental, historical, and modeled approaches, have delved into understanding climate change across the Holocene era and millennial-scale occurrences. However, the chronology and causes of centennial-scale climate events during the Holocene remain controversial. In this study, we overviewed 10 of the best-resolved and most accurately dated records detailing climate change in the Northern Hemisphere (NH) over the Holocene, obtained from different proxies across different climatic zones, and constructed a stack of temperature changes in the NH. Based on the constructed stack, we identified and categorized 15 notable Holocene centennial cooling events (HCCEs) in the NH (period with temperature decreases). To test the chronological validity of the constructed HCCEs, we compared them with the most accurately dated and highly resolved climate records during the last 3 kyr, which have been extensively investigated by the scientific community. Based on the close alignment of the outlined HCCEs with temperature records, we suggest that other HCCEs also match centennial climate cooling events over the last 10 kyr. To understand the origins of the established HCCEs, we compared them with potential climate influencing factors: total solar irradiance (TSI), explosive volcanic activity, Atlantic meridional overturning circulation (AMOC)-limited slowdowns, Intertropical Convergence Zone (ITCZ) fluctuations, and El Niño/Southern Oscillation (ENSO variability. Early Holocene HCCE 5, terminated by a prominent 8.2-ka cold event, was likely driven by the superposition of the AMOC limited slowdown, TSI minimum, and volcanic activity. The Holocene Thermal Maximum (HTM) happened between HCCEs 5 and 4a and was interrupted by HCCE 4c and 4b, coeval, with a significant southward shift of the ITCZ, likely related to cooling in the tropical zone. However, the sequence of HCCEs 3b, 3a, and 2b (over 4.53–3.42 BP), accompanied by small changes in the TSI, was likely forced by an increase in ENSO variability, leading to remarkable changes in the tropical processes and a southward shift of the ITCZ, coeval with the collapse of the Chinese Neolithic cultures and onset of the Holocene Neoglacial. Subsequent HCCEs 2a–0a were likely forced by the TSI minimum combined with the influence of ENSO and volcanism over the last 2 ka.

Millennial to centennial-scale climate oscillations since 15000 cal yrs BP from Kanwar wetland in the Central Ganga Plain, India

Millennial-scale climate cycles have been meagerly discussed in the terrestrial records from the Indian subcontinent, particularly the wetlands of the Ganga Plain. The mineral magnetic and textural analyses, along with chronology, distinctly record variation in the monsoon intensity since 15000 cal yrs BP in the sediment core collected from the Kanwar Lake in the Central Ganga Plain (CGP) of India. This unique high-resolution data allowed us to analyse monsoon fluctuation in the CGP with respect to changes in the North Atlantic process (internal) and external (solar cycles) forcing. During this period, several palaeoclimatic events, such as the Bølling-Allerød (B/A), Older and Younger Dryas, and the high-frequency fluctuations in the Holocene viz., Greenlandian, Northgrippian, and Meghalayan stages, were observed. The B/A event is correlated with the Indian Summer Monsoon (ISM) intensification period of the CGP. Eleven dry events are recorded at 13600–13200, 13000–11700, 11000, 10600, 9800–9000, 8200, 6100–5300, 4300–4200, 3800, 1800–900 and 300 cal yrs BP. During the Holocene, the sustained episodes of weaker monsoon centered around 10600, 9600, 8200, 6000, 4200, 2800, 1400 and 600 cal yrs BP, where the variability of the proxy data is mostly synchronous with the reduced upwelling intensity in the western Arabian Sea as well as roughly align with ∼1500 years cyclic signals in the drifted ice proxy recorded Bond Events indicating weakened ISM circulation during colder North Atlantic. Two distinct short cycles, spanning 172–344 and 172–688 years, are evident around 12,000 and 14,000 cal yrs BP with an additional occurrence from 86 to 344 years during 2000 and 6000 cal yrs BP, affirming the significance of external forcing, particularly solar activity. For the 1492 years cycle in ISM fluctuation, we postulate it may represent a combined forcing of external (solar) and internal (Atlantic Meridional Overturning Circulation (AMOC)) factors during the Holocene. This is the first high-resolution record available in the CGP, which brings out crucial monsoon reconstruction during the last 15000 years.

Marine Isotope Stage 11c in Europe: Recent advances in marine–terrestrial correlations and their implications for interglacial stratigraphy – a review

The interglacial known as MIS 11c (c. 426 000–396 000 years ago) receives intensive international interest because of its perceived role as an analogue for the current interglacial and its importance for understanding future climate change. Here we review the current understanding of the stratigraphy of this interglacial in Europe. This study considers (i) the evidence for the environmental history of this interglacial as reconstructed from the varved lake records from northern Europe, (ii) the climate history of MIS 11c as preserved in the long pollen records of southern Europe and (iii) a comparison of both of these with marine records from the North Atlantic. The result of this review is a discussion of the evidence for millennial and centennial scale climate change found in European records of MIS 11c, the patterns of warming that are seen across this interglacial and the discrepancy in aspects of the duration of this interglacial that seems to exist between the marine and terrestrial records of this warm period. A review of the recent advances in the study of MIS 11c in Europe confirms its importance for understanding both the past evolution of the Holocene and the future patterns of long‐term climate change.

Assessing atmospheric and oceanic teleconnections between the eastern and western Mediterranean over the past 8000 years

Holocene climate records from the Mediterranean are marked by pervasive millennial to centennial-scale climate variability. Here, we investigate East-West Mediterranean atmospheric and oceanic teleconnections by computing phase-relationships between oxygen isotope (δ18O) records generated on Soreq (East) and Chorchia (West) spelaeothems, as well as between δ18O and carbon isotope (δ13C) records from planktonic and benthic foraminifera from core PS009PC (East, Levantin Basin), ODP Site 963D (Central, Sicily Strait), and core KESC9-14 (West, Ligurian Basin). These marine sites are all located at intermediate water depths (560–460 m depth). Hence, the benthic foraminiferal δ18O records reflect mainly the intermediate ocean temperature/δ18O of the water mass, and the benthic δ13C is a proxy for the intensity of water flowing at the studied depth called Levantine Intermediate Water (LIW). For both western and eastern cores, the planktonic stable isotopic records reflect the climate-induced activity of the nearby river system. We find broadly in-phase relationships between the spelaeothem δ18O records and between the planktonic δ18O and δ13C records at most multi-centennial and millennial periodicities. This is indicative of closely linked (hydro-) climatic conditions in Southern Europe, the Levant, and North Africa over the last 8000 years. Conversely, at intermediate water depths, we find a distinct out-of-phase relationship between the East/Central and West Mediterranean benthic δ18O and δ13C records at 1000–2000 years periodicities. We interpret this see-saw pattern as indicative of a persistent regional influence of LIW on oceanographic conditions in the intermediate depths of the eastern basin. Conversely, we suggest a strong influence of the modified Atlantic Ocean inflow (MAW) in the intermediate water formation in the Western Mediterranean (‘Winter Intermediate Water’; WIW). This WIW overprints, at least partially, the LIW signal that reaches the western Mediterranean causing the out-of-phase relationship between the east and the west oceanographic signals at intermediate depths.

Relationship between the rise and fall of Loulan ancient city and centennial-scale climate events and cycles

Relationship between the rise and fall of Loulan ancient city and centennial-scale climate events and cycles

Exploring the Centennial-Scale Climate History of Southern Brazil with Ocotea porosa (Nees & Mart.) Barroso Tree-Rings

This article explores the dendrochronological potential of Ocotea porosa (Nees & Mart) Barroso (Imbuia) for reconstructing past climate conditions in the General Carneiro region, Southern Brazil, utilizing well-established dendroclimatic techniques. A total of 41 samples of Imbuia were subjected to dendroclimatic analysis to reconstruct precipitation and temperature patterns over the period from 1446 to 2011. Notably, we achieved the longest reconstructions of spring precipitation and temperature for the Brazilian southern region, spanning an impressive 566-year timeframe, by employing a mean chronology approach. To achieve our objectives, we conducted a Pearson’s correlation analysis between the mean chronology and the climatic time series, with a monthly temporal resolution employed for model calibration. Impressively, our findings reveal significant correlations with coefficients as high as |rx,P| = 0.32 for precipitation and |rx,T| = 0.45 for temperature during the spring season. Importantly, our climate reconstructions may elucidate a direct influence of the El Niño—South Oscillation phenomenon on precipitation and temperature patterns, which, in turn, are intricately linked to the natural growth patterns of the Imbuia trees. These results shed valuable light on the historical climate variability in the Southern Brazil region and provide insights into the climatic drivers affecting the growth dynamics of Ocotea porosa (Nees & Mart) Barroso.

Lunar eclipses illuminate timing and climate impact of medieval volcanism

Explosive volcanism is a key contributor to climate variability on interannual to centennial timescales1. Understanding the far-field societal impacts of eruption-forced climatic changes requires firm event chronologies and reliable estimates of both the burden and altitude (that is, tropospheric versus stratospheric) of volcanic sulfate aerosol2,3. However, despite progress in ice-core dating, uncertainties remain in these key factors4. This particularly hinders investigation of the role of large, temporally clustered eruptions during the High Medieval Period (HMP, 1100–1300 ce), which have been implicated in the transition from the warm Medieval Climate Anomaly to the Little Ice Age5. Here we shed new light on explosive volcanism during the HMP, drawing on analysis of contemporary reports of total lunar eclipses, from which we derive a time series of stratospheric turbidity. By combining this new record with aerosol model simulations and tree-ring-based climate proxies, we refine the estimated dates of five notable eruptions and associate each with stratospheric aerosol veils. Five further eruptions, including one responsible for high sulfur deposition over Greenland circa 1182 ce, affected only the troposphere and had muted climatic consequences. Our findings offer support for further investigation of the decadal-scale to centennial-scale climate response to volcanic eruptions.

Structural similarity between Termination III and I

The sequence of abrupt climate variability during Termination III (T-III) is still controversial due to the scarcity of well-dated and high-resolved paleoclimate records. Here, we present 230Th-dated stalagmite δ13C and δ18O data covering T-III from northern China for the first time, with an average resolution of ∼20 years. T-III was featured by two Weak Monsoon Intervals (WMIs), separated by a millennial-scale interstadial. The earlier and later WMI was labeled as Mystery Interval-III (MI-III) and Younger Dryas-III (YD-III), respectively, and the interstadial as Bølling-Allerød-III (BA-III). The transition of MI-III/BA-III, BA-III/YD-III and YD-III/PI (Penultimate Interglacial), constrained by lamination counting, lasted 52, 87 and 68 years, respectively. According to 230Th dating, the duration of the climate variability is 3253 ± 2835, 1373 ± 2294 and 1266 ± 2309 years, respectively. If the KLS-20 δ13C record was aligned to Sanbao cave record within chronology uncertainties, the best estimates of these durations are ∼2300, ∼2100 and ∼4000 years, respectively. Additionally, two significant centennial-scale climate rebounds within YD-III were identified in our records, comparable to the three short-lived climate excursions within YD observed in stalagmite BW-1 δ13C and δ18O records from the same cave and NGRIP δ18O record. The close coupling of abrupt climate oscillations during T-III and T-I highlights the structural similarity between the two deglaciations, both of which are characterized by multi-interruption of WMI. It is probably due to the relatively low amplitude and rate of northern summer insolation rise during T-III and T-I, compared with T-II and T-IV.

Spatio-temporal patterns of centennial-scale climate change over the Tibetan Plateau during the past two millennia and their possible mechanisms

In order to project future changes in climate and their corresponding environmental and social impacts, it is of utmost importance to understand the spatiotemporal patterns of past climate changes on the Tibetan Plateau (TP). Although numerous records of late Holocene climate changes have been developed across the TP, the spatiotemporal patterns of changes in temperature and precipitation/moisture are unclear and a comprehensive review of this topic is urgently needed. Here, we present such a synthesis using currently available temperature and precipitation/moisture records from across the TP to understand spatial and temporal variations over the past two millennia. The main conclusions are as follows: 1) Temperature variations on the TP during the past two millennia are not homogeneous and show strong seasonality. Although patterns of mean annual temperature change on the TP during the past two millennia remain contradictory, summer temperature variations are generally consistent. The summer temperature reconstructions indicate that the warmest period during the past two millennia was in the middle ages, as opposed to the recent 20th century. 2) Precipitation/moisture patterns on the TP are spatially complex. The most striking feature is that an inverse pattern of precipitation/moisture variation can be detected over most parts of the TP between 600-1400 AD and 1400-1900 AD. Specifically, the eastern TP, southwestern TP, and part of the northeastern TP were generally wetter, whereas the southeastern TP, the southern TP, the northwestern TP, and part of the northeastern TP were generally drier during 600–1400 AD as compared to during 1400–1900 AD. However, such anti-phased hydroclimatic variability is not found in either the cold period of 1–600 AD or in the current warm period of the past century. This indicates that temperature and precipitation/moisture changes appear in multiple combinations. 3) Fluctuations in the intensity and seasonal movement of the westerlies and the Indian summer monsoon (ISM) over the Tibetan Plateau, as well as the associated climate variability modes (e.g., North Atlantic Oscillation (NAO), Indian Ocean dipole (IOD), ENSO, and Intertropical Convergence Zone (ITCZ)), might be responsible for the spatially complex patterns of climate anomalies over TP during the past 2000 years. 4) The results highlight the need for more high-quality paleoclimatic and paleoenvironmental reconstructions that span the past two millennia on the TP. Such reconstructions must have unambiguous climate significance, clear seasonality, site-specific calibration, and robust dating.

Late Holocene vegetation, climate, and lake changes in northern China: Varved evidence from western Loess Plateau

High-resolution archives deepen the understanding of past climate variability. We report new sedimentological and paleoecological data from Chaona Lake in northern China. The record represents the annually laminated (i.e., varved) archive from the western Loess Plateau spanning the Late Holocene, allowing insights into critical time intervals of decadal to centennial-scale climate instability. After developing a robust, continuously chronology supported by radioisotope dating and varve chronology, we used high-resolution palynological and sedimentological data to decipher the specific climate and ecosystem evolution over the last 2800 years. Our results show a general forest decline and climate deterioration intercalated with a series of oscillations during the Late Holocene, which may have profoundly influenced the eco-social development of northern China. In addition, lake development changes that mainly reflect the transformation from deep to shallow lake conditions generally match the regional vegetation, which is probably driven by climate-related processes. However, fluctuations in well or poor and the absence of varved sediment indicate variations in the water circulation in the lake catchment. Periods of predominantly well-varved sediments are considered to reflect reduced lake circulation and more anoxic conditions, coinciding with warmer and more stable climate intervals, such as the Roman Warm Period and the Medieval Warm Period. Conversely, periods of poor or even non-varved preservation indicate strengthened lake circulation, which may be influenced by strong winds (e.g., 2800–2000 cal yr BP) and cold/drought conditions (e.g., the Little Ice Age). Integration of our data with those of published climatic reconstructions in northern and eastern monsoon China suggests that this variability in climate can be explained by shifts in solar insolation and large-scale ocean-atmospheric coupling dynamics that affect the Loess Plateau (e.g., the East Asia Summer Monsoon and El Niño-Southern Oscillation).

Indian summer monsoon variations during the Younger Dryas as revealed by a laminated stalagmite record from the Tibetan Plateau

High-resolution and precisely dated hydroclimate records in the southeastern Tibetan Plateau (TP) remain sparse beyond the Holocene, which hampers our ability to understand the hydroclimate variability in this important Indian summer monsoon (ISM) fringe area and its global teleconnection. Here we present 3-y resolution δ18O and δ13C records from a laminated stalagmite (RG-3) from Rige Cave in the southeastern TP, spanning the Younger Dryas (YD). The records allow us to precisely characterize the timing, structure, and particularly centennial-scale events within the YD, and probe the control factors of precipitation δ18O (δ18Op) in the ISM fringe area. On centennial–millennial timescales, the Rige δ18O record shows coherent pattern with the East Asian summer monsoon (EASM) counterpart, combined with modeled δ18Op results and spatial analysis, suggesting that δ18Op in this part of the TP is largely controlled by the large-scale atmosphere circulation (or the ISM strength), and the altitude increase in the TP may not potentially result in an opposite δ18Op pattern at least in the monsoonal TP regime. We also found significant δ13C-δ18O covariation on the centennial timescale, suggesting a coincided ISM rainfall and biomass change during the YD. In the Rige records, one weak centennial-scale ISM event (namely the intra-Allerød cold period, IACP) and three strong centennial-scale ISM events within the YD (namely A1′-A2′-A3′) were prominent and occurred between ∼12,470 and 12,310 ± 14, ∼12,210–12,090 ± 12 and ∼12,010–11,920 ± 12 y BP (before present, where present = 1950 CE), respectively. Spectral analyses of Rige records also revealed a significant ∼200-y periodicity, which is nearly in-phase with observed centennial-scale variations of the North Atlantic temperature and mid-latitude westerly-jet during that time. These observations support the hypothesis that the solar de Vries cycle (207-y) triggered the centennial-scale climate variations in high northern latitude, leading to the ISM variations via fast atmospheric processes.

Hydroclimate proxies for eastern Australia using stable isotopes in grey mangroves (Avicennia marina)

The development of high-resolution terrestrial palaeoclimate records in Australia is hindered by the scarcity of tree species suitable for conventional dendrochronology. However, novel analytical techniques have made it possible to obtain climate information from tree species that do not reliably form annual growth rings. In this paper we assess the potential of stable carbon and oxygen isotopes in the xylem wood of grey mangroves (Avicennia marina (Forssk.) Vierh.) as hydroclimate proxies for eastern Australia. Bomb-pulse radiocarbon dating and simple age models were used to estimate the age of the growth layers in radial sequence in stems from four grey mangrove trees in two adjacent estuaries in New South Wales, Australia. Stable isotope data measured from the xylem wood of the four stems were composited to yield mean δ18O and δ13C series for the 1962–2016 period. Significant negative Spearman correlations were found between δ18O and rainfall, sea level, instrumental Palmer Drought Severity Index (scPDSI) and the El Niño Southern Oscillation (ENSO), while δ13C was positively correlated with temperature, vapour pressure and evapotranspiration. The results demonstrate that stable oxygen isotopes in grey mangroves have the potential to yield valuable information about pre-instrumental hydroclimate. Grey mangroves can survive with intact centres for an estimate of >250 years based on observed growth rates, are widespread along northern Australian and tropical coastlines and could provide important information regarding pre-instrumental climate in regions currently lacking high-resolution (i.e., near annual) centennial scale climate proxy records.

Centennial-scale climate variabilities during the Holocene on northeastern Tibetan Plateau

Centennial-scale climate variabilities during the Holocene on northeastern Tibetan Plateau

Late Holocene Mongolian climate and environment reconstructions from brGDGTs, NPPs and pollen transfer functions for Lake Ayrag: Paleoclimate implications for Arid Central Asia

A coupled pollen-brGDGT paleoclimate reconstruction approach has been tested to provide independent and robust estimates of Holocene climate and environment changes in the extremely arid environment of the mountainous areas ranging from northern Arid Central Asia (ACA) to the Mongolian Plateau. The two proxies were calibrated for both global and local modern data sets (NMSDB). This multi-proxy approach was then applied to a sediment core collected from Lake Ayrag, Arkhangai, covering the Late Holocene. In addition to brGDGTs and pollen, we also performed magnetic susceptibility, micro-XRF, elemental and isotopic bulk chemistry, and Non-Pollen Palynomorph (NPP) analyses on the Lake Ayrag sediments in order to better understand the lake system and human impact dynamics. While the globally calibrated record (both for pollen and brGDGTs) displayed a slight millennial-scale cooling, the locally calibrated results exhibit centennial-scale climate oscillations such as the 4.2 and 3.5 kyr events, the Roman Warm Period (RWP), Dark Ages Cold Period (DACP), Medieval Warm Period (MWP) and Little Ice Age (LIA). These climate oscillations and vegetation changes are discussed with regard to the main Mongolian human historical occupation events documented by pastoralism proxies, especially the Xiongnu, Mongol Empire, Mandchou and Soviet periods. The climate systems currently dominating the Mongolian Plateau are difficult to resolve because inter-annual climate variability is pronounced. However, precipitation mainly occurs in summer (easterly monsoon driven) when the winter Westerlies lead the air mass movement. In the past, both pollen and biomarkers exhibited anti-correlated trends with annual precipitation and temperature: over the last 4000 kcal yr BP, the warm periods (MWP, RWP) were dry and the cold periods (LIA, DACP, 3.5 kyrs) were humid. Thus, the East Asian Summer Monsoon (i.e., warm and wet conditions dominant during summer) seems not to have influenced central Mongolian climate during the Late Holocene, which could have remained dominated by the Westerlies/Siberian High cells conflict. A comparison between the Ayrag record and other paleoclimate records from the Baikal area (Dulikha), Mongolian Plateau (D3L6, D1L1, NRX, ATM), and continental China (Kesang, Baluk and Tonnel caves, XRD section) to the Loess Plateau (Huangye and Xianglong caves) suggests that the monsoon front has oscillated since the Early Holocene. A climate synthesis following strictly the same approach (locally calibrated brGDGTs vs. pollen-inferred climate) for all the ACA records available for the Late Holocene helps us to resolve the climate systems paced by centennial to millennial-scale oscillations and their consequences for human societies.

Centennial scale climate oscillations from southern Siberia in the Last Glacial Maximum

A lack of adequate high resolution climate proxy records for the Last Glacial Maximum (LGM) has prevented the extrapolation of climate–solar linkages on centennial time scales prior of the Holocene. Therefore, it is still unknown whether centennial climate variations of the last ten thousand years convey a universal climate change or merely represent a characteristic of the Holocene. Recently published high resolution climate proxy records for the LGM allowed us to extrapolate climate–solar linkages on centennial time scales ahead of the Holocene. Here we present the analysis of a high resolution pollen concentration record from Lake Kotokel in southern Siberia, Russia, during the LGM. The record reflects the dynamics of vegetation zones and temperature change with a resolution of ∼40 years in the continental climate of north-eastern Asia. We demonstrate that our pollen concentration record, the oxygen isotope δ18O record from the Greenland ice core project NGRIP (NorthGRIP), the dust-fall contributions in Lake Qinghai, China, grain size in the Gulang and Jingyuan loess deposits, China, and the composite oxygen isotope δ18O record from the Alpine cave system 7H reveal cooler to warmer climate fluctuations between ∼ 20.6 and 26 ka. Such fluctuations correspond to the ∼1000-yr, 500-600-yr and 210-250-yr cycles possibly linked to the solar activity variations and recognized in high resolution Holocene proxies all over the world. We further show that climate fluctuations in the LGM and Holocene are spectrally similar suggesting that linkages between climate proxies and solar activity at the centennial time scale in the Holocene can be extended to the LGM.

Tree-Ring Width Data of Tsuga longibracteata Reveal Growing Season Temperature Signals in the North-Central Pearl River Basin since 1824 AD

Concerning the ecological and economical importance of the Pearl River basin, short-term climate changes have been widely studied by using the instrumental records in the basin, but there is still a lack of long-term climatic reconstructions that can be used to evaluate the centennial scale climate anomalies. Here, we present a 237-year tree-ring width chronology from Tsuga longibracteata in the north-central Pearl River basin, with reliable coverage from 1824 to 2016. Based on the significant relationship between tree growth and mean temperature from the previous March to the previous October, we reconstructed the previous growing season (pMar-pOct) temperatures for the past 193 years, with an explained variance of 43.3% during 1958–2016. The reconstruction reveals three major warm (1857–1890, 1964–1976, and 1992–2016) and cold (1824–1856, 1891–1963, and 1977–1991) periods during 1824–2016. Comparison with other temperature sensitive proxy records from nearby regions suggests that our reconstruction is representative of large-scale temperature variations. Significant correlations of tree growth with the sea surface temperatures (SSTs) in the western Pacific Ocean, northern Indian Ocean, and Atlantic Ocean suggest that SST variability in these domains may have strongly influenced the growing season temperature change in the Pearl River basin.

Response of bay ostracod assemblages to Late Holocene sea-level, centennial-scale climate, and human-induced factors in northeast Beppu Bay, Japan

This study clarified the centennial-scale changes in meiobenthic bay ostracod assemblages in Japan over the past approximately 3000 years with relation to various human-induced and natural environmental factors. These factors were inferred via integrated multiproxy methods of high-resolution geological analyses of core sediments obtained from a shallow bay off a coastal plain, along with literature surveys of archeological and historical records. Five intervals were defined based on multivariate analyses of ostracod assemblages. The ostracod assemblage was stable before around the 6th century because of aggradational sedimentation related to gradual increases in sea level. Since then, the composite factors such as the development of a sandy spit near the study site, stable sea level, regional tectonics, regional centennial-scale climatic change possibly related to El Niño Southern Oscillation (ENSO), and flood mitigation by human settlement in the coastal plains triggered the formation of tidal flats and subtidal sandy areas with seagrass beds. This has markedly influenced offshore bay ostracod assemblages since around the 12th century. Anthropogenic impacts, such as reclamation and various artificial constructions since the late 20th century caused the disappearance of seagrass beds, input of coarser sediment into offshore bays, and increased nutrient loads. Therefore, ostracod assemblages have changed drastically. Ostracod assemblages near the boundary between land and sea have been affected by multiple complex factors, such as regional climate and depositional and human-induced processes during the Late Holocene.

Unmixing deep-sea paleoclimate records: A study on bioturbation effects through convolution and deconvolution

Marine sediments are typically subject to bioturbation from benthic macrofauna, which disrupt sediments and can have a substantial impact on the amplitude and timing of climate proxy signals recorded in marine sediment cores. This study explores a computational approach to quantify and remove these post-depositional effects from marine climate proxy records. The bioturbation process is modeled as a linear time-invariant filter building upon prior work (Guinasso and Schinck, 1975). Using forward modeling we find that, given modeled mixing layer depths ≥ 5 cm, simulated centennial scale climate variability is generally not preserved even when the sedimentation rate in our model is above 15 cm/kyr. On the scale of ice ages (104-105 yr), the observed effects of bioturbation have the smallest impact, considering the event scale and typical sedimentation rates and mixed layer depth in the deep sea. For millennial scale events, the signal attenuation strongly depends on the event scale and specific bioturbation and sedimentation parameters. To account for the bioturbation effect on a given climate proxy series, a deconvolution method is proposed. We apply the approach to three individual benthic foraminifera oxygen isotope records to reconstruct δ18O across the last interglacial (Marine Isotope Stage 5e; MIS 5e), and to assess the magnitude of the deglacial shift between MIS 6 and MIS 5e. In the highest resolution records (Site MD952042, Site 1012), the deconvolution results reveal an increase in the amplitude of the recovered deglacial δ18O shift by approximately 0.1‰, with implications for reconstructed global average sea level variations and temperature estimates from benthic foraminifera δ18O. The recovered signal at Site 677 suggests high amplitude δ18O variability during MIS 5e, but exhibits larger uncertainty due to a lower sampling frequency, lower sedimentation rate, and an inferred higher noise level. While uncertainties on the deconvolution results can be large, the results provide a new view of the potential impact of bioturbation on paleoclimate reconstruction. Future work can reduce the uncertainty in the deconvolution estimates, through quantitative integration of additional constraints, provided by our knowledge of the climate and depositional systems.

Climate Variability in Central Europe during the Last 2500 Years Reconstructed from Four High-Resolution Multi-Proxy Speleothem Records

The Late Holocene was characterized by several centennial-scale climate oscillations including the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period and the Little Ice Age. The detection and investigation of such climate anomalies requires paleoclimate archives with an accurate chronology as well as a high temporal resolution. Here, we present 230Th/U-dated high-resolution multi-proxy records (δ13C, δ18O and trace elements) for the last 2500 years of four speleothems from Bunker Cave and the Herbstlabyrinth cave system in Germany. The multi-proxy data of all four speleothems show evidence of two warm and two cold phases during the last 2500 years, which coincide with the Roman Warm Period and the Medieval Warm Period, as well as the Dark Ages Cold Period and the Little Ice Age, respectively. During these four cold and warm periods, the δ18O and δ13C records of all four speleothems and the Mg concentration of the speleothems Bu4 (Bunker Cave) and TV1 (Herbstlabyrinth cave system) show common features and are thus interpreted to be related to past climate variability. Comparison with other paleoclimate records suggests a strong influence of the North Atlantic Oscillation at the two caves sites, which is reflected by warm and humid conditions during the Roman Warm Period and the Medieval Warm Period, and cold and dry climate during the Dark Ages Cold period and the Little Ice Age. The Mg records of speleothems Bu1 (Bunker Cave) and NG01 (Herbstlabyrinth) as well as the inconsistent patterns of Sr, Ba and P suggests that the processes controlling the abundance of these trace elements are dominated by site-specific effects rather than being related to supra-regional climate variability.

The impact of cyclical, multi-decadal to centennial climate variability on arsenic sequestration in lacustrine sediments

Examining paleoclimate-driven changes of elemental contaminants, such as Arsenic (As), increases the understanding of the mobility and fate of elements under a warming climate scenario. To characterize the variability in As sequestration in the sediments of a freshwater system in response to decadal- to centennial-scale climate oscillations, a freeze-core (CON01) was recovered from Control Lake, Northwest Territories. Radiocarbon dating of 13 bulk-organic samples provided temporal reference to core depth. Sediment geochemistry was determined using Itrax X-ray fluorescence core-scanning (Itrax-XRF). Elemental concentrations were measured on a sub-set of samples using ICP-MS after multi-acid (MA) digestion to assess the accuracy of Itrax-XRF results through a multivariate log-ratio (MLC) calibration. Comparison of Itrax-XRF to ICP-MS using the MLC in ItraXelerate software show Pearson's R2 values >0.75, with the exception of As (R2 = 0.44). MLC-calibrated Itrax-XRF elemental data were centered log-ratio (CLR) transformed to eliminate issues related to data closure. During the ca. 3300-yr sedimentary record, moderate-strength negative correlations between AsCLR and KCLR (Spearman's ρ = −0.38, p-value < 0.001, n = 785), and AsCLR and TiCLR, (Spearman's ρ = −0.52, p-value < 0.001, n = 785) suggest that As is primarily sequestered in sediments during intervals of warmer temperatures and higher productivity. Proxies for sediment particle size (TiCLR, KCLR) and As concentration (AsCLR) were examined for response to quasi-periodic climate oscillations using spectral analysis. Significant periodicities were observed with approximately 4–13, 30–60, 90–120, and 160–280 yr periods in TiCLR, KCLR, and AsCLR records. These frequencies are interpreted as corresponding to the North Atlantic Oscillation and/or 8–14-yr Schwabe sunspot cycles, 30–60-yr Pacific Decadal Oscillation, and centennial-scale solar cycles (e.g., 90-yr Gleissberg cycle; 205-yr Suess cycle). Coeval occurrence of these periodicities revealed through wavelet analysis of Control Lake geochemistry data suggests that these climate cycles only impact Control Lake when they occur concurrently.