Description: There is compelling evidence that episodic deposition of large volumes of fresh water into the oceans strongly influenced global ocean circulation and climate variability during glacial periods1,2. In the North Atlantic region, episodes of massive freshwater discharge to the North Atlantic Ocean were related to distinct cold periods known as Heinrich Stadials1–3. By contrast, the freshwater history of the North Pacific region remains unclear, giving rise to persistent debates about the existence and possible magnitude of climate linkages between the North Pacific and North Atlantic oceans during Heinrich Stadials4,5. Here we find that there was a strong connection between North Atlantic circulation changes during Heinrich Stadials and injections of fresh water from the North American Cordilleran Ice Sheet to the northeastern North Pacific. Our record of diatom 18O (a measure of the ratio of the stable oxygen isotopes 18O and 16O) over the past 50,000 years shows a decrease in surface seawater 18O of two to three per thousand, corresponding to a decline in salinity of roughly two to four practical salinity units. This coincided with enhanced deposition of ice-rafted debris and a slight cooling of the sea surface in the northeastern North Pacific during Heinrich Stadials 1 and 4, but not during Heinrich Stadial 3. Furthermore, results from our isotope-enabled model6 suggest that warming of the eastern Equatorial Pacific during Heinrich Stadials was crucial for transmitting the North Atlantic signal to the northeastern North Pacific, where the associated subsurface warming resulted in a discernible freshwater discharge from the Cordilleran Ice Sheet during Heinrich Stadials 1 and 4. However, enhanced background cooling across the northern high latitudes during Heinrich Stadial 3—the coldest period in the past 50,000 years7—prevented subsurface warming of the northeastern North Pacific and thus the increased freshwater discharge from the Cordilleran Ice Sheet. In combination, our results show that nonlinear ocean–atmosphere background interactions play a complex role in the dynamics linking the freshwater discharge responses of the North Atlantic and North Pacific during glacial periods.

Description: A new 21.3m firn core was drilled in 2015 at a coastal Antarctic high-accumulation site in Adélie Land (66.78◦ S; 139.56◦ E, 602 m a.s.l.), named Terre Adélie 192A (TA192A). The mean isotopic values (−19.3 ‰ ± 3.1 ‰ for δ18O and 5.4 ‰±2.2 ‰ for deuterium excess) are consistent with other coastal Antarctic values. No significant isotope–temperature relationship can be evidenced at any timescale. This rules out a simple interpretation in terms of local temperature. An observed asymmetry in the δ18O seasonal cycle may be explained by the precipitation of air masses coming from the eastern and western sectors in autumn and winter, recorded in the d-excess signal showing outstanding values in austral spring versus autumn. Significant positive trends are observed in the annual d-excess record and local sea ice extent (135–145◦ E) over the period 1998–2014. However, process studies focusing on resulting isotopic compositions and particularly the deuterium excess–δ18O relationship, evidenced as a potential fingerprint of moisture origins, as well as the collection of more isotopic measurements in Adélie Land are needed for an accurate interpretation of our signals.

Description: The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ18O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ18O is most similar to the amount-weighted precipitation δ18O where mean annual temperature (MAT) is 〈 10 °C. By contrast, for seasonal climates with MAT 〉 10 °C and 〈 16 °C, drip water δ18O records the recharge-weighted δ18O. This implies that the δ18O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.

Description: The Southern Ocean (SO) has long been recognised as a key player in regulating atmospheric CO2 variations and hence global climate. Here, the biological utilisation of nutrients regulates the preformed nutrient inventory for most of the deep ocean and, therefore, the global average efficiency of the biological pump. Marine sediment records from the Subantarctic Atlantic and Pacific document that higher mineral dust flux, increased bioproductivity, and lower atmospheric CO2 co-varied on glacial-interglacial time scales, which has been associated with iron fertilisation. It has been suggested that up to 40-50 ppmv of past atmospheric CO2 changes are related to iron fertilisation in the Subantarctic Ocean. The main objective of DustIron is an improved characterisation of the modern and past dust cycle and its link to SO iron fertilisation and atmospheric CO2 through a closely coupled novel datamodel approach. Within this project we want to extend the available geographic coverage of modern dust deposition, provenance and marine productivity records as well as the spatial and temporal variability during past glacial-interglacial cycles across all SO sectors. For the modern ocean we will explore dust fluxes, grain-size, and geochemical source area fingerprints (iron mineralogy, isotopy). Iron fertilisation and productivity will be assessed with a variety of both traditional (e.g., fluxes of biogenic barium or opal) and novel proxies for nutrient utilisation (δ15N in foraminifera). Our paleostudies will focus on the last glacial-interglacial climate transitions from the western Atlantic proximal to the Patagonian sources and from the central Indian SO (Kerguelen) to the eastern Indian Ocean sector, in order to obtain a circum-Antarctic view of dust-productivity changes. This will be complemented by a modelling study, simulating glacial-interglacial changes of atmospheric dust concentrations, deposition fluxes and linked SO bioproductivity.

Description: Stable oxygen and hydrogen isotope records from ice cores represent the major climate proxy archive for the reconstruction of past Antarctic temperatures. This is based on observed relations between variations in the isotopic composition of precipitation and variations in local air temperature, which arise from the temperature-dependent distillation and fractionation of the water vapour isotopic composition along its atmospheric trajectory. Additionally, other processes influence the isotopic composition recorded in ice cores, such as variable atmospheric circulation and moisture pathways, the intermittent nature of precipitation, and mixing and re-location of snow by wind drift. These processes lead to variable relationships between isotopes and temperature across sites and add noise to the temperature proxy record. Available key Antarctic temperature reconstructions of the last two millennia therefore rely on spatial averages of a large number of cores to reduce overall noise. However, the efficiency of the noise reduction in this averaging step depends on the spatial structure of the temperature signal and the other processes. While sound knowledge is available for the noise and its spatial structure in isotope records sampled on a local (~1 km) scale (Münch et al., 2016; Laepple et al., 2017), so far we lack sufficient information on the spatial structure of the processes important on the larger scales, e.g. precipitation intermittency and atmospheric circulation changes. Here, we analyse the millennium simulation of the ECHAM-wiso isotope-enabled general circulation model in order to learn about the spatio-temporal structure of the temperature-to-isotope relationship and its impact on the climate variability recorded in spatial arrays of ice-core isotope records. Using the climate model simulation output and our process model for the signal formation in ice cores, we estimate optimal core positions to reconstruct the temperature at a target location and also evaluate the best use of existing ice cores. Our results provide new insights into the ability of ice-core isotope records to reconstruct past regional temperature variability in Antarctica and into the spatial scales of the processes that drive the isotope variability depending on the topographic setting. Thereby, they lay the foundations for future ice-core drilling efforts in order to improve and extend present Antarctic climate reconstructions of the last two millennia. Laepple, T., et al., The Cryosphere, 12(1), 169–187, doi: 10.5194/tc-12-169-2018, 2017. Münch, T., et al., Clim. Past, 12(7), 1565–1581, doi: 10.5194/cp-12-1565-2016, 2016.

Description: Although quantitative isotope data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to using the speleothem data for data–model comparisons. Here, we illustrate this using 456 globally distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates the process of procuring large numbers of records if data–model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotope values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model’s ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotope data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on δ18O values, the optimum period for the modern observational baseline and the selection of an appropriate time window for creating means of the isotope data for palaeo-time-slices.