Marit-Solveig Seidenkrantz

Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years.

Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation.

Evidence for solar forcing of sea-surface temperature on the North Icelandic shelf during the late Holocene

Diatom proxies from the modern position of the oceanographic Polar Front north of Iceland record variability in sea-surface temperatures (SSTs) during the past 2 k.y. The sedimentary record is dated with tephrochronology, alleviating marine 14C reservoir age uncertainties. Comparison of changes in SSTs on the North Icelandic Shelf with variations in the atmospheric circulation above Greenland, North American Atlantic coastal SSTs, and mean temperature anomalies for the Northern Hemisphere suggests synchronous North Atlantic–wide fluctuations, which would seem to imply a common forcing factor. A positive and significant correlation between our SST record from the North Icelandic Shelf and reconstructed solar irradiance, together with modeling results, supports the hypothesis that solar forcing is an important constituent of natural climate variability in the northern North Atlantic region.

Diatom surface sediment assemblages around Iceland and their relationships to oceanic environmental variables

Canonical correspondence analysis of diatoms from surface sediment samples and oceanographic environmental variables shows that summer and winter sea-surface temperatures, water depth and winter sea-surface salinity are the main environmental factors affecting diatom distribution around Iceland. Of these, summer sea-surface temperature is the most important. Five diatom assemblages are distinguished and the distribution of these assemblages is clearly correlated with oceanic current patterns in the region. The sea-ice diatom assemblage is limited to the area where the East Greenland Current (Polar Water) has its strongest influence, and the cold diatom assemblage is basically controlled by the less cold East Icelandic Current (Modified Polar Water). The mixing diatom assemblage results from the interaction between the cold East Greenland and East Icelandic Currents and the warm Irminger Current. The warm diatom assemblage is located in the area dominated by the Irminger Current and may be used as an indicator of warm-water masses (Atlantic Water). The coastal diatom assemblage is the only one strongly influenced by both water depth and summer water temperatures.

Deep-water changes: the near-synchronous disappearance of a group of benthic foraminifera from the Late Miocene Mediterranean

Abstract Benthic foraminiferal and geochemical data from the Monte del Casino section in northern Italy are employed to reconstruct the sequence of events preceding the Late Miocene Mediterranean salinity crisis. We evaluate the effects of eustatic sea-level changes and tectonic events, affecting the Atlantic–Mediterranean connections. Changing benthic foraminiferal assemblages record changes in the deep-water environment, that can be explained by progressive isolation of the Mediterranean basin. The results of the analyses of stable oxygen- and carbon isotopes and redox-sensitive elements are in line with the benthic faunal trends. At 7.16 Ma, a first major step in isolation of the basin is indicated by the nearly simultaneous disappearance of a group of deeper-water benthic species usually found in middle to lower slope environments. At the same time, stable isotopes and redox-sensitive elements indicate cooling and decreasing bottom water oxygenation. After 6.8 Ma gradual development of water-mass stratification, probably accompanied by increasing bottom-water salinity, is indicated by all proxies. The cyclic pattern of homogeneous and sapropelitic sediments in the section is related to astronomical parameters, and allows a detailed correlation with Mediterranean sections at considerable geographic distances. We compared the benthic foraminiferal faunas with those from the Metochia section (Gavdos, Greece), deposited at a similar water depth (∼1000 m), to find that the benthic foraminifera at both sites indicate a similar, synchronous paleoenvironmental development. All proxies indicate that the onset of the Late Miocene Mediterranean salinity crisis dates back to the earliest Messinian. The development of the Mediterranean basin toward an evaporite trap is a result of interactions between a 400-kyr eccentricity related climatic effect superimposed upon gateway dynamics at the Atlantic–Mediterranean connections. Benthic foraminifera accurately record the steps in increasing isolation of the Mediterranean basin.

MODERN DISTRIBUTION OF BENTHIC FORAMINIFERA ON THE NORTH ICELANDIC SHELF AND SLOPE

Forty-six surface samples from the north Icelandic shelf and slope were analyzed with respect to both living (stained) and total (living/stained + dead/unstained) benthic foraminiferal faunas. Near-coastal samples are strongly dominated by species indicating a high-energy environment, among these various species of the genus Cibicides. Nonionellina labradorica, which has a strong affinity to areas of high surface primary production, is constrained to oceanic boundaries on the outer shelf. Faunistically, the area is further divided into eastern and western parts, the submarine Kolbeinsey Ridge forming a barrier. Calcareous species, particularly Melonis barleeanus, prevail in the western part, while the assemblages in the eastern part are strongly dominated by agglutinated foraminifera. Principal Component Analyses (PCA) help distinguish four main components of both the total (living + dead) assemblage (TA) and the total living assemblage (TLA), as well as of the total (living + dead) calcareous assemblage (CA) and the living calcareous assemblage (CLA). The components of the TA and TLA are similar, the major differences being controlled by species-dependent variables such as adaptability to changes in food supply and, in the eastern part of the area, depth of microhabitat and possibly a limited primary production. The PCA analyses of the calcareous species alone (CA and CLA) define components with significant differences in the foraminiferal population. The calcareous distribution in particular is important in application of modern distribution patterns to paleoceanographical and paleoclimatological reconstructions in areas where post-mortem disintegration of the agglutinated fauna may have occurred.

Benthic foraminifera as indicators of changing Mediterranean–Atlantic water exchange in the late Miocene

Abstract Benthic foraminifera and stable isotopes from homogeneous sediments in an Upper Miocene (Tortonian–lower Messinian) cyclic succession of homogeneous marls and sapropels from the Island of Gavdos (Greece) show significant changes in bottom-water oxygenation, presumably related to changes in the Mediterranean–Atlantic water exchange. The benthic foraminifera prove to be excellent indicators of variations in the oxygen contents of the bottom waters, even more so than the stable isotopes. Our data indicate a step-wise restriction of the Mediterranean–Atlantic water exchange resulting in increasing sluggishness of deep-water circulation. The first indication of restricted circulation is seen at 8.5 Ma, which may correspond to a severe restriction of the external zone of the Betic Strait. A second event, which had a very serious influence on the fauna in the deeper Mediterranean, occurred at 7.16 Ma. This event was presumably linked to the partial closure of the Rifian Corridor. The restricted Mediterranean–Atlantic connection also seems to have led to a gradual salinity increase and possibly to the formation of a brine in the deepest basins, thus marking the first development towards the Messinian Salinity Crisis.

Evidence for external forcing of the Atlantic Multidecadal Oscillation since termination of the Little Ice Age

The Atlantic Multidecadal Oscillation (AMO) represents a significant driver of Northern Hemisphere climate, but the forcing mechanisms pacing the AMO remain poorly understood. Here we use the available proxy records to investigate the influence of solar and volcanic forcing on the AMO over the last ~450 years. The evidence suggests that external forcing played a dominant role in pacing the AMO after termination of the Little Ice Age (LIA; ca. 1400–1800), with an instantaneous impact on mid-latitude sea-surface temperatures that spread across the North Atlantic over the ensuing ~5 years. In contrast, the role of external forcing was more ambiguous during the LIA. Our study further suggests that the Atlantic Meridional Overturning Circulation is important for linking external forcing with North Atlantic sea-surface temperatures, a conjecture that reconciles two opposing theories concerning the origin of the AMO.

Taking the pulse of the Sun during the Holocene by joint analysis of 14C and 10Be

We have studied solar variations during the Holocene (i.e., last similar to 11,700 yr) by combining a new model of the Earths dipole moment with C-14 data from the IntCal04 record and 10 Be data from the GRIP ice core. Joint spectral analysis of the two nuclide records suggests that the periodic behavior of the Sun was particularly pronounced between 6000-4500 yr BP and 3000-2000 yr BP, with dominating periodicities of similar to 88, similar to 150, similar to 220, and similar to 400 years, while this rhythmic behavior faded during other time intervals. The fact that the two reconstructions, based on radionuclides with distinct geochemical properties, agree with respect to both the frequency and timing of the periodic behavior, strongly suggests that they reflect the actual behavior of the Sun. Subtle but systematic differences between the amplitude spectra may point to an interplay between the climate system and the similar to 220- and similar to 400-year solar cycles during intervals when these were particularly prominent. Citation: Knudsen, M. F., P. Riisager, B. H. Jacobsen, R. Muscheler, I. Snowball, and M.-S. Seidenkrantz (2009), Taking the pulse of the Sun during the Holocene by joint analysis of 14 C and 10 Be, Geophys. Res. Lett., 36, L16701, doi: 10.1029/2009GL039439. (Less)

Cassidulina teretis Tappan and Cassidulina neoteretis new species (Foraminifera): stratigraphic markers for deep sea and outer shelf areas

Fossil versus Recent specimens allocated to Cassidulina teretis Tappan display slight differences in both ecological distribution and morphology. This has led to a re-examination of specimens from the North Atlantic region. The study was mainly based on scanning electron microscopy. It resulted in the division of the plexus into two species with different stratigraphical and partly different environmental distributions: Cassidulina teretis Tappan and Cassidulina neoteretis n. sp. A distinction between C. teretis and C. neoteretis provides a new biostratigraphical marker and may also prove useful in ecological studies. C. teretis had its first occurrence during the Middle to Upper Miocene and its last well-documented occurrence shortly after the palaeomagnetic Brunhes/Matuyama boundary in the Norwegian Sea, whilst it apparently disappeared from the North Atlantic as early as a little after the Gauss/Matuyama boundary. C. neoteretis presumably evolved from C. teretis between about 2.0 and 2.3 Ma in the northern North Atlantic and migrated northward inhabiting the Norwegian Sea as C teretis became extinct here at about 0.7 Ma. All Recent specimens belong to C. neoteretis. Specimens of C. teretis have been documented from both arctic and boreal regions in inner shelf to bathyal environments (between about 50 and 2000 m water depth), whereas the Recent distribution of C. neoteretis is slightly more limited: arctic and cold boreal regions at water depths between 150 and 3000 m, most commonly between 1000 and 1500 m.

Solar forcing of Holocene summer sea-surface temperatures in the northern North Atlantic

Mounting evidence from proxy records suggests that variations in solar activity have played a significant role in triggering past climate changes. However, the mechanisms for sun-climate links remain a topic of debate. Here we present a high-resolution summer sea-surface temperature (SST) record covering the past 9300 yr from a site located at the present-day boundary between polar and Atlantic surface-water masses. The record is age constrained via the identification of 15 independently dated tephra markers from terrestrial archives, circumventing marine reservoir age variability problems. Our results indicate a close link between solar activity and SSTs in the northern North Atlantic during the past 4000 yr; they suggest that the climate system in this area is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation. Furthermore, the high-resolution SST record indicates that climate in the North Atlantic regions follows solar activity variations on multidecadal to centennial time scales.