Thorsten Kiefer

Glacial North Atlantic: Sea‐surface conditions reconstructed by GLAMAP 2000

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX-28) modern analog technique with 947 modern analog samples and 119 well-dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice-free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45degrees and 60degreesN, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12degreesC. In the subtropical ocean gyres the new reconstruction supports the glacial-to-interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2degrees and 6degreesN suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6degrees-8degreesC during Northern Hemisphere summer. Differences between these SIMMAX-based and published U37(k)- and Mg/Ca-based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.

Fundamental Modes and Abrupt Changes in North Atlantic Circulation and Climate over the last 60 ky — Concepts, Reconstruction and Numerical Modeling

Centennial- to millennial-scale changes in global climate over the last 60 ky were first documented in ice cores from Greenland, with ice sheets around the North Atlantic and its thermohaline circulation (THC) as prime candidates for a potential trigger mechanism. To reach a new quality in understanding the origin and causal links behind these changes, two strategies were intimately tied together in this synthesis, high-resolution 3-D ocean modeling and paleoceanographic reconstructions. Here, five time series with a time resolution of several decades and various time slices of surface and deep-water paleoceanography were established from hundreds of deep-sea cores for the purpose of monitoring rapid changes across the North Atlantic and testing or initiating model results. Three fundamental modes were found to operate Atlantic THC. Today, mode I shows intensive formation of North Atlantic Deep Water (NADW) and strong heat and moisture fluxes to the continents adjacent to the North Atlantic. Peak glacial mode II leads to a reduction in NADW formation by 30-50%, in line with a clear drop in heat flux to Europe. The glacial Nordic Seas, however, remain ice-free during summer and little influenced by meltwater, in contrast to the sea west ofIreland, where iceberg meltwater blocks an eastbound flow into the Norwegian Sea and induces a cold longshore current from Faeroe to the Pyrenees. The subsequent Heinrich 1 (HI) meltwater mode III leads to an entire stop in NADW and intermediate-water production as well as a reversed pattern of THC, stopping any heat advection from the central and South Atlantic to the north. In contrast to earlier views, the Younger Dryas, possibly induced by Siberian meltwater, began with mode I and ended with mode III, continuing into the Preboreal. Modeling the impact of modes I to III on the global carbon budget, we find that the atmosphere has lost 34-54 ppmv CO2 from interglacial to glacial times, but has gained 23-62 ppmv CO2 at the end of HI within a few decades, equivalent to 33-90% of modem, man-made CO2 release. The robust 1500-y Dansgaard- Oeschger (D-O) cycles and their multiples of as much as 7200 years, the Heinrich event cycles, are tied to periodical changes between THC modes I/II and II/III. In the Irminger Sea rapid D-O coolings are in phase with initial meltwater injections from glaciers on East Greenland, here suggesting an internal trigger process in accordance with binge-purge models. Ice rafting from East Greenland and Iceland occurs only 240-280 y later, probably inducing a slight sea-level rise and, in tum, Heinrich ice rafting from the Laurentian ice sheet during H1, H2, H4, H5. At H1 a major surge from the Barents shelf has lagged initial cooling by 1500 y and entails the most prominent and extended reversal in Atlantic THC over the last 60 ky (probably also at the end of glacial stage 4, at H6). Meltwater stratification in the Inninger Sea reaches its maximum only 640 y after initial meltwater injection and induces, via seasonal sea-ice formation, brine-water injections down to 4 km water depth, signals leading the classic D-O jump to maximum warmth by only 125 y. It may be inferred from this short-phase lag that brine water-controlled deep-water formation probably entrains warm water from further south, thereby forming the key trigger mechanism for the final tum-on of the Atlantic THC mode II roughly within a decade (or mode I, in case of favorable Milankovitch forcing).

A review of nitrogen isotopic alteration in marine sediments

Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth Abstract: Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.

Looking forward through the past : identification of 50 priority research questions in palaeoecology

Summary 1. Priority question exercises are becoming an increasingly common tool to frame future agendas in conservation and ecological science. They are an effective way to identify research foci that advance the field and that also have high policy and conservation relevance. 2. To date there has been no coherent synthesis of key questions and priority research areas for palaeoecology, which combines biological, geochemical and molecular techniques in order to reconstruct past ecological and environmental systems on timescales from decades to millions of years. 3. We adapted a well-established methodology to identify 50 priority research questions in palaeoecology. Using a set of criteria designed to identify realistic and achievable research goals, we selected questions from a pool submitted by the international palaeoecology research community and relevant policy practitioners. This article is protected by copyright. All rights reserved. Accepted Article 4. The integration of online participation, both before and during the workshop, increased international engagement in question selection. 5. The questions selected are structured around six themes: human–environment interactions in the Anthropocene; biodiversity, conservation, and novel ecosystems; biodiversity over long timescales; ecosystem processes and biogeochemical cycling; comparing, combining and synthesizing information from multiple records; and new developments in palaeoecology. 6. Future opportunities in palaeoecology are related to improved incorporation of uncertainty into reconstructions, an enhanced understanding of ecological and evolutionary dynamics and processes, and the continued application of long-term data for better-informed landscape management. 7. Synthesis Palaeoecology is a vibrant and thriving discipline and these 50 priority questions highlight its potential for addressing both pure (e.g. ecological and evolutionary, methodological) and applied (e.g. environmental and conservation) issues related to ecological science and global change.

North Pacific response to millennial‐scale changes in ocean circulation over the last 60 kyr

Foraminifera-based sea surface temperature estimates in the northwest Pacific (Ocean Drilling Program Site 883; 51°N) varied by 4°C on millennial timescales over the last 60,000 calendar (cal) years, with the most prominent amplitudes during marine isotope stage 3. Age control is based on benthic δ18O records, 14C ages, and on geomagnetic intensity variations at Site 883 tuned to the North Atlantic paleointensity stack since 75 ka (NAPIS-75), in turn, tied to the Greenland Ice Sheet Project 2 (GISP2) ice core chronology. On the basis of this tuning, northwest Pacific warm phases parallel Greenland cold stadials and viceversa. This contrasts with atmospheric heat transfer, expected to produce quasi-coeval signals across the Northern Hemisphere. The antiphasing may instead stem from variations in global thermohaline circulation. At its North Pacific terminus in the subarctic gyre, any slowdown or shutoff of North Atlantic Deep Water formation during Dansgaard-Oeschger stadials led to a turnoff or reduction of the upwelling of cold Pacific deepwater and thus, to striking, instantaneous, short-term warmings of surface water.

Paleonutrient and productivity records from the subarctic North Pacific for Pleistocene glacial terminations I to V

Our study addresses fundamental questions of the mode and timing of orbital and millennial-scale changes in the meridional overturning circulation (MOC) of the subarctic North Pacific. Particular concerns are the vertical mixing, the present and past abundance of nutrients in surface waters despite strong stratification, and the North Pacific-North Atlantic seesaw of oscillations in sea surface temperature (SST). We do this by generating and interpreting multiple records for glacial terminations I-V down two long piston cores, one each from the western and eastern subarctic Pacific. Chlorins and biogenic opal are proxies for surface water productivity; delta(13)C of epibenthic foraminifera is a record of deepwater ventilation; and the delta(13)C of N. pachyderma sin. is a tracer of nutrients in subsurface waters that extend up to the sea surface during times of vertical mixing. The degree of mixing is traced by pairing SST and delta(18)O records of planktic surface and subsurface (pycnocline) dwellers. Tight age control is deduced from a suite of age-calibrated (14)C plateau boundaries for Termination I and benthic delta(18)O and geomagnetic events for the last 800 ka. Carbon 14 paleoreservoir ages record the ages of surface and deep waters to uncover short-term changes in MOC over Termination I. We have defined a standard sequence of short-term productivity events for Termination I, also evident during terminations II to V and subsequent interglacials over the last 450 ka. The peak glacial regime of stable stratification and low productivity terminated, together with the end of ice rafting and melting, near 17 ka, similar to 2000 years after the onset of Termination I. Pulses of vertical mixing and incursion of warm surface waters from the subtropics followed. Convected young water masses finally penetrated down to 3600-m water depth at 17.0 to less than 14.5 ka, significantly improving bottom water ventilation through the late deglacial and earliest interglacial. Mixing with upwelled nutrients from the pycnocline induced short-term maxima in algal production of chlorins and biogenic opal near 17-15 and 15-12 ka, respectively. Deglacial meltwater incursions in the Aleutian Current and silica input from North American rivers also promoted East Pacific productivity after 15.5 ka. Productivity decreased during the late deglacial and early interglacial, coeval with an exceptional peak in CaCO(3) preservation caused by both low organic flux and well-ventilated deepwater. Subsequently, low salinity and cool surface waters and in turn, stratification were gradually restored. A second, opal-dominated productivity maximum marked the ends of interglacials. The deglacial pulses of vertical mixing around 17-11 ka imply an important contribution of the North Pacific to the coeval release of oceanic CO(2) into the atmosphere and support the east-west seesaw model of climate change.

A Review of 2000 Years of Paleoclimatic Evidence in the Mediterranean

The integration of climate information from instrumental data and documentary and natural archives; evidence of past human activity derived from historical, paleoecological, and archaeological records; and new climate modeling techniques promises major breakthroughs for our understanding of climate sensitivity, ecological processes, environmental response, and human impact. In this chapter, we review the availability and potential of instrumental data, less well-known written records, and terrestrial and marine natural proxy archives for climate in the Mediterranean region over the last 2000 years. We highlight the need to integrate these different proxy archives and the importance for multiproxy studies of disentangling complex relationships among climate, sea-level changes, fire, vegetation, and forests, as well as land use and other human impacts. Focusing on dating uncertainties, we address seasonality effects and other uncertainties in the different proxy records. We describe known and anticipated challenges posed by integrating multiple diverse proxies in high-resolution climate-variation reconstructions, including proxy limitations to robust reconstruction of the natural range of climate variability and problems specific to temporal scales from interannual to multicentennial. Finally, we highlight the potential of paleo models to contribute to climate reconstructions in the Mediterranean, by narrowing the range of climate-sensitivity estimates and by assimilating multiple proxies.

Temporal changes in North Atlantic circulation constrained by planktonic foraminiferal shell weights

Records of planktonic foraminiferal shell weights for Globigerina bulloides, covering Termination I from four proximal sites at waters depths from 1150 to 4045 m in the northeast Atlantic, demonstrate the influence of dissolution superimposed upon initial shell weight variability. Records of shell weight, unaffected by dissolution, may be used as a reference for interpreting deeper records in terms of preservation history. Combining records of planktonic shell weight (and benthic δ13C) from shallow and deep sites suggests that maximum oceanic stratification and incursion of southern sourced deep waters in the North Atlantic occurred at about 18–20 ka, defining the glacial mode of Atlantic circulation. Reduced stratification and enhanced preservation in deeper waters reflect conditions during Heinrich event 1. A state similar to the modern mode of Atlantic circulation was attained by about 10 ka.

Warmings in the far northwestern Pacific promoted pre-Clovis immigration to America during Heinrich event 1

Well-dated multidecadal- to centennial-scale sediment records from the subarctic northwest Pacific show that the early deglacial 18.5–15.0 ka was marked by 3 pronounced short-term warmings of ~5 °C. They lasted 500–1500 yr each and were coeval with early to late stages of cold Heinrich event 1 in the North Atlantic. These regional climate windows may have promoted a pre-Clovis emigration of people from the cold-arid monsoon climate in East Asia to the climatically more favorable, then-emerged Beringian and Aleutian shelf regions and the Americas, as suggested by archeological findings.

Antarctic control on tropical Indian Ocean sea surface temperature and hydrography

[1] We reconstructed the surface hydrography of the South Equatorial Current in the western Indian Ocean for the last 65,000 years using a marine sediment core record. Results show that tropical Indian Ocean temperatures resemble temperatures from Antarctic ice cores with warm and cold fluctuations synchronous with the Antarctic Cold Reversal and the Antarctic warm events A1–A4. The most likely thermal link involves Subantarctic Mode Water (SAMW) which forms north of the subpolar frontal zone and spreads northward into the Indian Ocean. This subsurface water mass is the prime suspect because of a stronger temperature response in the thermocline (recorded by the foraminifer N. dutertrei) than in surface water (G. ruber).