Low-frequency Pliocene climate variability in the eastern Nordic Seas

The Pliocene (5.3–2.6 Ma) is often described as a relatively stable climatic period, with warm temperatures characterizing high latitudes. New suborbital resolved stable isotope records from ODP Hole 642B in the Eastern Nordic Seas document that the Pliocene was not a stable period characterized by one climate. Rather, seven distinct climate phases, each lasting between 150,000 and 400,000 years, are identified and characterized in the time interval 5.1–3.1 Ma. Four of the transitions between the defined climate phases occurred close to an eccentricity minimum and a minimum in amplitude of change for Northern Hemisphere summer insolation, while two occurred around an eccentricity maximum and a maximum in amplitude in insolation change. Hence, a low frequency response of the Nordic Seas to insolation forcing is indicated. In addition, paleogeographic and related paleoceanographic changes, expansion of the Arctic sea ice cover and onset of NHG were important factors behind the evolving Pliocene low frequency variability in the eastern Nordic Seas. It is likely that the identified climate phases and transitions are important beyond the Nordic Seas, due to their association with changes to both insolation and paleogeography. Also, a strong and variable degree of diagenetic calcite overgrowth is documented for the planktic foraminifera, especially influencing the planktic δ18O results; the absolute values and amplitude of change cannot be taken at face value

Co-feeding of live feed and inert diet from first-feeding affects Artemia lipid digestibility and retention in Senegalese sole (Solea senegalensis) larvae

The present study intended to evaluate the effects of early introduction of inert diet in lipid digestibility and metabolism of sole, while larval feed intake, growth and survival were also monitored. Solea senegalensis larvae were reared on a standard live feed regime (ST) and co-feeding regime with inert diet (Art R). Trials using sole larvae fed with Artemia enriched with two different lipid emulsions, containing glycerol tri [1-14C] oleate (TAG) and L-3-phosphatidylcholine-1,2-di-[1-14C] oleoyl (PL), were performed at 9 and 17 days after hatching (DAH) to study lipid utilization. Co-feeding did not affect sole survival rates (ST 59.1 ± 15.9 %; Art R 69.56 ± 9.3 %), but was reflected in significantly smaller final weight at 16 DAH (ST 0.71 ± 0.20; Art R 0.48 ± 0.14 mg). Higher feed intake was observed in sole larvae fed on Artemia enriched with labeled PL at 9 DAH but not at 17 DAH. At 17 DAH, the smaller larvae (Art R treatment) ingested proportionally more Artemia in weight percentage, independently of enrichment. At 9 DAH lipid digestibility was equal among treatments and higher than 90%, while at 17 DAH it was higher in ST treatment (around 73 %) compared to the Art R group (around 66 %). Lipid retention efficiency at 9 DAH was higher in the Art R treatment, reaching values of 50 %, while these values almost duplicated at 17 DAH, ranging up to 80 % in both treatments without significant differences. These results show that co-feeding of live feed and inert diet from first-feeding in Senegalese sole has a toll in terms of growth and lipid digestibility but does not seem to compromise lipid metabolic utilization

The role of the Barents Sea in the Arctic climate system

Present global warming is amplified in the Arctic and accompanied by unprecedented sea ice decline. Located along the main pathway of Atlantic Water entering the Arctic, the Barents Sea is the site of coupled feedback processes that are important for creating variability in the entire Arctic air-ice-ocean system. As warm Atlantic Water flows through the Barents Sea, it loses heat to the Arctic atmosphere. Warm periods, like today, are associated with high northward heat transport, reduced Arctic sea ice cover, and high surface air temperatures. The cooling of the Atlantic inflow creates dense water sinking to great depths in the Arctic Basins, and ~60% of the Arctic Ocean carbon uptake is removed from the carbon-saturated surface this way. Recently, anomalously large ocean heat transport has reduced sea ice formation in the Barents Sea during winter. The missing Barents Sea winter ice makes up a large part of observed winter Arctic sea ice loss, and in 2050, the Barents Sea is projected to be largely ice free throughout the year, with 4°C summer warming in the formerly ice-covered areas. The heating of the Barents atmosphere plays an important role both in “Arctic amplification” and the Arctic heat budget. The heating also perturbs the large-scale circulation through expansion of the Siberian High northward, with a possible link to recent continental wintertime cooling. Large air-ice-ocean variability is evident in proxy records of past climate conditions, suggesting that the Barents Sea has had an important role in Northern Hemisphere climate for, at least, the last 2500 years

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

A comprehensive multi-proxy study on two sediment cores from the western and central Skagerrak was performed in order to detect the variability and causes of marine primary productivity changes in the investigated region over the last 1100 years. The cores were dated by Hg pollution records and AMS 14C dating and analysed for palaeoproductivity proxies such as total organic carbon, δ13C, total planktonic foraminifera, benthic foraminifera (total assemblages as well as abundance of Brizalina skagerrakensis and other palaeoproductivity taxa) and palaeothermometers such as Mg∕Ca and δ18O. Our results reveal two periods with changes in productivity in the Skagerrak region: (i) a moderate productivity at  ∼ &thinsp;CE&thinsp;900–1700 and (ii) a high productivity at  ∼ &thinsp;CE&thinsp;1700–present. During  ∼ &thinsp;CE&thinsp;900–1700, moderate productivity was likely driven by the nutrients transported with the warm Atlantic water inflow associated with a tendency for a persistent positive NAO phase during the warm climate of the Medieval Climate Anomaly, which continues into the LIA until  ∼ &thinsp;CE&thinsp;1450. The following lower and more variable temperature period at  ∼ &thinsp;CE&thinsp;1450–1700 was likely caused by a reduced contribution of warm Atlantic water, but stronger deep-water renewal, due to a generally more negative NAO phase and a shift to the more variable and generally cooler climate conditions of the Little Ice Age. The productivity and fluxes of organic matter to the seafloor did not correspond to the temperature and salinity changes recorded in the benthic Melonis barleeanus shells. For the period from  ∼ &thinsp;CE&thinsp;1700 to the present day, our data point to an increased nutrient content in the Skagerrak waters. This increased nutrient content was likely caused by enhanced inflow of warm Atlantic water, increased Baltic outflow, intensified river runoff, and enhanced human impact through agricultural expansion and industrial development. Intensified human impact likely increased nutrient transport to the Skagerrak and caused changes in the oceanic carbon isotope budget, known as the Suess effect, which is clearly visible in our records as a negative shift in δ13C values from  ∼ &thinsp;CE&thinsp;1800. In addition, a high appearance of S. fusiformis during the last 70 years at both studied locations suggests increased decaying organic matter at the sea floor after episodes of enhanced primary production.</p

ForCenS-LGM: a dataset of planktonic foraminifera species assemblage composition for the Last Glacial Maximum

Species assemblage composition of marine microfossils offers the possibility to investigate ecological and climatological change on time scales inaccessible using conventional observations. Planktonic foraminifera - calcareous zooplankton - have an excellent fossil record and are used extensively in palaeoecology and palaeoceanography. During the Last Glacial Maximum (LGM; 19,000 – 23,000 years ago), the climate was in a radically different state. This period is therefore a key target to investigate climate and biodiversity under different conditions than today. Studying LGM climate and ecosystems indeed has a long history, yet the most recent global synthesis of planktonic foraminifera assemblage composition is now nearly two decades old. Here we present the ForCenS-LGM dataset with 2,365 species assemblage samples collected using standardised methods and with harmonised taxonomy. The data originate from marine sediments from 664 sites and present a more than 50% increase in coverage compared to previous work. The taxonomy is compatible with the most recent global core top dataset, enabling direct investigation of temporal changes in foraminifera biogeography and facilitating seawater temperature reconstructions

Group 2i Isochrysidales produce characteristic alkenones reflecting sea ice distribution

AbstractAlkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s. However, alkenone-based SST reconstructions in the northern high latitude oceans show significant bias towards warmer temperatures in core-tops, diverge from other SST proxies in down core records, and are often accompanied by anomalously high relative abundance of the C37 tetra-unsaturated methyl alkenone (%C37:4). Elevated %C37:4 is widely interpreted as an indicator of low sea surface salinity from polar water masses, but its biological source has thus far remained elusive. Here we identify a lineage of Isochrysidales that is responsible for elevated C37:4 methyl alkenone in the northern high latitude oceans through next-generation sequencing and lab-culture experiments. This Isochrysidales lineage co-occurs widely with sea ice in marine environments and is distinct from other known marine alkenone-producers, namely Emiliania huxleyi and Gephyrocapsa oceanica. More importantly, the %C37:4 in seawater filtered particulate organic matter and surface sediments is significantly correlated with annual mean sea ice concentrations. In sediment cores from the Svalbard region, the %C37:4 concentration aligns with the Greenland temperature record and other qualitative regional sea ice records spanning the past 14 kyrs, reflecting sea ice concentrations quantitatively. Our findings imply that %C37:4 is a powerful proxy for reconstructing sea ice conditions in the high latitude oceans on thousand- and, potentially, on million-year timescales.</jats:p

Sea surface temperature variability in the Norwegian Sea during the late Pliocene linked to subpolar gyre strength and radiative forcing

The mid-Piacenzian warm period (3.264–3.025 Ma) of the Pliocene epoch has been proposed as a possible reference for future warm climate states. However, there is significant disagreement over the magnitude of high latitude warming between data and models for this period of time, raising questions about the driving mechanisms and responsible feedbacks. We have developed a new set of orbital-resolution alkenone-based sea surface temperature (SST) and ice rafted debris (IRD) records from the Norwegian Sea spanning 3.264–3.14 Ma. The SSTs in the Norwegian Sea were 2–3 °C warmer than the Holocene average, likely caused by the radiative effect of higher atmospheric CO2 concentrations. There is notable obliquity-driven SST variability with a range of 4 °C, shown by evolutive spectra. The correlation of SST variability with the presence of IRD suggests a common climate forcing acting across the Nordic Seas region. Changes of the SST gradient between the Norwegian Sea and North Atlantic sites suggest that the subpolar gyre was at least as strong as during the Holocene, and that the northward heat transport by the North Atlantic Current was comparable

Late Eemian warming in the Nordic Seas as seen in proxy data and climate models

We analyze a transient simulation of the last glacial inception in a climate model of intermediate complexity, focusing on sea ice-ocean circulation dynamics in the North Atlantic and Nordic Seas. As northern high-latitude summer insolation decreases toward the end of the Eemian interglacial, Arctic sea ice export to the North Atlantic increases. This surface fresh water transport weakens deep water formation in the North Atlantic and the near-surface circulation of the subpolar gyre. As a consequence, the relative contribution of subpolar gyre waters to the Atlantic inflow into the Nordic Seas is reduced, giving way to more warm and saline subtropical waters from the North Atlantic Current. We thus find an episode of relatively high heat and salt transport into the Nordic Seas during the last glacial inception between 119,000 and 115,000 years before present. This stabilizes deep ocean convection in the region and warms Scandinavia during a phase of low insolation. These findings are in good agreement with proxy data from the Nordic Seas and North Atlantic. At the end of the warm interval, sea surface temperature drops by about 3 degrees C, marking the onset of large-scale glacier growth over Scandinavia

Sea surface temperatures and ice rafting in the Holocene North Atlantic: climate influences on Northern Europe and Greenland

The oceanographic conditions in the high-latitude North Atlantic ocean during the Holocene were reconstructed through analyses of sea surface temperature (SST; alkenone unsaturation ratios) and ice rafting (mineralogy and grain size) from two sediment sequences, one recovered from the Reykjanes Ridge at 59degreesN and the other from the Norwegian Sea at 68degreesN. Comparison of our records to published ice core and terrestrial proxy-climate data sets suggests that atmospheric temperature changes over Northern Europe and Greenland were coupled to SST variability and ice rafting. The records outline four major climatic phases: (i) an early-Holocene Thermal Maximum that lasted until approximately 6.7 kyr BP, (ii) a distinctly cooler phase associated with increased ice rafting between 6.5 and 3.7 kyr BP, (iii) a transition to generally warmer, but relatively unstable climate conditions between 3.7 and 2 kyr BP and (iv) a second distinct SST decline that took place between 2 and 0.5 kyr BP. In contrast to the dominant control of Northern Hemisphere summer insolation on early-Holocene climate development (via strong seasonality), the trigger for the onset of relatively unstable climatic conditions in the North Atlantic at 3.7 kyr BP is not straightforward. However, it is possible that this change was triggered by late-Holocene winter insolation increase at high northern latitude and/or by inter-hemispheric changes in orbital forcing. The late-Holocene Neoglaciation trend, which is characteristic of numerous terrestrial archives in northern Europe, may not only be attributed to a gradual decrease in orbitally forced summer temperature, but also to increase snow precipitation at high northern latitudes during generally milder winters. (C) 2004 Elsevier Ltd. All rights reserved