Coupling of palaeoceanographic shifts and changes in marine reservoir ages off North Iceland through the last millennium

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Abstract

Our study of spatial and temporal marine reservoir age changes on the North Icelandic shelf is based on two high-resolution 1000-year sedimentary records from IMAGES core sites MD99-2273 and MD99-2275, located at 650 m and 470 water depths, respectively. The sedimentary and fossil record is extremely sensitive to past oceanographic and climatic changes in this region. It has been demonstrated that the position of the Polar Front across the shelf has been very dynamic through the last millennium, and our results show that deviations in the marine reservoir ages can be used as a palaeoceanographic tool for tracing changes in the water masses. The presence of historically and terrestrially dated tephra markers from Icelandic volcanoes makes it possible to construct a reliable tephrochronological age-depth model for each of the marine sediment cores. A comparison of these age models with age models based on calibrated AMS radiocarbon age determinations of marine molluscs displays major deviations through the last 1000 years. The discrepancy between the two types of age models shows that there was an average reservoir age of about 650 years at MD99-2273 and 590 years at site MD99-2275, with considerable changes through time. This is suggested to be related to a generally high, but fluctuating, inflow of apparently old Polar waters to the area. A comparison with the distribution of two benthic foraminiferal indicator species in the cores, one for Polar and Arctic water masses (Islandiella norcrossi) and one for Atlantic Water (Cassidulina neoteretis), as well as with different proxies for sea-ice cover in the area, shows that there was an increase in the penetration of cold Polar and Arctic water masses after AD 1300, i.e. during the Little Ice Age, at both sites, resulting in an increase in the reservoir ages. Relatively high reservoir ages throughout the last millennium at the deepest core site (MD99-2273) can be attributed to a continuously high Polar Water influence at the sea floor. The location of this site to the west of the Kolbeinsey Ridge, which has acted as a submarine barrier throughout the Holocene, may have contributed to a relatively high influence of cold Denmark Strait overflow water in this area.

Introduction

The North Icelandic shelf (Fig. 1A, B) is an area of pronounced oceanographic sensitivity to climatic changes because of the vicinity to the strong North Atlantic oceanographic and atmospheric climatic fronts that separate temperate and arctic climatic conditions (Johannessen, 1986, Ólafsson, 1999). The North Icelandic shelf is characterized by a series of sedimentary basins, some of them with extremely high sedimentation rates (MD99-2273 c. 600 cm/1000 years; MD99-2275 c. 300 cm/1000 years), allowing sufficient temporal resolution for important information on past oceanographic and climatic changes. The environmental changes in this region have been intensively studied through the last decade, often with special emphasis on the last few millennia, e.g. by Eiríksson et al., 2000, Eiríksson et al., 2006, Andrews et al., 2001a, Andrews et al., 2001b, Andrews et al., 2001c, Jiang et al., 2002, Jiang et al., 2005, Jiang et al., 2007, Jennings et al., 2001, Knudsen and Eiríksson, 2002, Andrews and Giraudeau, 2003, Knudsen et al., 2004, Knudsen et al., 2009, Massé et al., 2008, Sicre et al., 2008a, Sicre et al., 2008b.

The North Icelandic shelf study area is located within a volcanically active region, and the sediments in cores MD99-2273 and MD99-2275 (Fig. 1B) contain several well-known historically dated tephra horizons. This contributes to a reliable age determination of the record by avoiding the 14C dating problem of temporal and spatial increase in the marine reservoir ages, related to incursions of Polar and Arctic water masses (cf. Larsen et al., 2002, Eiríksson et al., 2004).

Oceans are depleted in their 14C content relative to the 14C content of the atmosphere, and the apparent 14C age difference between the oceanic water and the contemporaneous atmosphere is called the marine reservoir age R (cf. Stuiver and Polach, 1977, Stuiver and Braziunas, 1993, Hughen et al., 2004, Reimer et al., 2009). The reservoir age R can vary locally, and deviations from the marine model average 400 yr reservoir age are widespread, both spatially and temporally (cf. Reimer and Reimer, 2001, Larsen et al., 2002, Eiríksson et al., 2004, Ascough et al., 2005, Matsumoto, 2007). These regional age offsets from the world ocean model 14C age are expressed as ΔR. A comparison of the tephrochronological age models with age models based on calibrated AMS radiocarbon age determinations of marine molluscs displays major deviations through the last 1000 years, indicating oceanographic changes at the core sites as a result of oscillations in the position of the Polar Front off North Iceland (e.g. Eiríksson et al., 2004).

The aim of this study is to demonstrate a coupling of changes in marine reservoir ages and palaeoceanographic shifts on the North Icelandic shelf with special focus on the last millennium, including the late part of the Medieval Warm Period (MWP) and the Little Ice Age (LIA). We present detailed tephrochronological age models for the core sites MD99-2273 and MD99-2275, based on an expanded number of tephra markers, with indication of age-model uncertainties.

The marine reservoir age deviations, calculated for each radiocarbon dated level in the cores, are compared with a series of high-resolution palaeoclimatic proxies, such as selected species of benthic foraminifera, ice-rafted debris (IRD), biomarker IP25 concentrations (Massé et al., 2008) and percentages of sea-ice diatom species (cf. Jiang et al., 2001, Ran et al., 2008, Ran et al., 2011-this issue) for the last 1000 years, as well as the long-term reconstructed NAO winter index (Trouet et al., 2009) and data from the Greenland ice-core records (Grootes et al., 1993, Dahl-Jensen et al., 1998, Stuiver and Grootes, 2000).

Section snippets

Oceanography

The marine Polar Front, which is characterized by steep temperature and salinity gradients, runs along the North Icelandic shelf (e.g. Ólafsson, 1999, Rytter et al., 2002). It separates the relatively warm high-salinity Atlantic Water, which is brought to the area by the Irminger Current, and the cold low-salinity modified Polar Water of the East Icelandic Current (Fig. 1A), called Arctic Surface Water (Johannessen, 1986). At present, the Arctic Surface Water north of Iceland consists of Polar

Instrumental and documentary data

Instrumental records from the northern North Atlantic have shown that this area has experienced oceanographic anomalies on a decadal time-scale in recent times (Fig. 2), the most prominent being the so-called salinity anomalies (Dickson et al., 1988, Belkin et al., 1998). During these periods, Arctic Ocean water was advected into the North Atlantic either via the Fram Strait or the Canadian Archipelago (Belkin et al., 1998). A strengthening of the East Greenland Current, and associated increase

Materials and methods

This study is based on material from piston cores MD99-2273 (66°45.78′N; 18°45.02′W; 650 m water depth) and MD99-2275 (66°33.10′N; 17°41.99′W; 440 m water depth) (Fig. 1B), located to the west and east of the Kolbeinsey Ridge, approximately 75 and 50 km offshore (Fig. 1B), respectively. The cores were obtained in 1999 during the MD114 IMAGES V cruise with the R/V Marion Dufresne (Labeyrie et al., 2003) (Fig. 1B). They were supplemented with multicore B05-2006-MC03B (66°45.79′N; 18°44.98′W; 660 m

Comparison of instrumental and proxy data

As a basis for our reconstruction of environmental changes through the last millennium, instrumental and documentary data from the North Icelandic shelf have been compared with proxy data from the same area (Fig. 2). Age models for the proxy data are based on a 210Pb dating of multicores (cf. Knudsen et al., 2009). Pearson's product-moment correlation coefficient was used to investigate linear trends between palaeoenvironmental proxies and instrumental and documentary records (Table 5). In

Chronology and reservoir ages

Age models for core sites MD99-2273 and MD99-2275 are constrained on the basis of the first appearance depth of each tephra marker and of the historical age or terrestrial radiocarbon date of the marker. The age-depth models are constructed using depositional models in OxCal 4.1 (Ramsey, 2008, Ramsey, 2009), and 210Pb dates of multicore B05-2006-MC04 at core site MD99-2275 are included in the age model for that site (cf. Knudsen et al., 2009). Most historical volcanic eruptions occur within a

Reservoir ages and benthic foraminifera

The possible relation between reservoir age changes off North Iceland and incursion of cold Arctic water masses into the area was previously discussed by Eiríksson et al. (2004). On the basis of a considerable expansion of the number of tephra markers, as well as new high-resolution proxy data from the last millennium at sites MD99-2273 and MD99-2275 (Fig. 1B), a detailed comparison is now possible, even for separate more restricted intervals within the last 1000 years (Fig. 5, Fig. 6).

The

Comparison and discussion

The temporal changes in the reservoir age deviations and in the benthic foraminiferal indication have been compared with other data from core MD99-2275, with particular focus on temperature and sea-ice proxies (Fig. 6), all reflecting the oceanographic changes in the area.

There is an interesting similarity between the I. norcrossi/C. neoteretis flux ratios and the IP25 concentration record (ends at AD 1895; Massé et al., 2008). In general, the values of both proxies are low during the MWP and

Summary and conclusion

Comparison of tephrochronologically based age-depth models for core sites MD99-2273 and MD99-2275 on the North Icelandic shelf with radiocarbon dates of molluscs from the same cores shows that there has been a spatial as well as a temporal change in the marine reservoir ages throughout the last millennium.

The oceanographic Polar Front is located across the shelf, and the sedimentological and fossil record in the area is extremely sensible to past climatic and oceanographic changes. The

Acknowledgements

This paper is a contribution to the European Unions 6th framework project MILLENNIUM (Contract No. EVK-CT-2006-017008). The project was additionally supported by grants from the Icelandic Research Council (Grant 050261031, ALDA), the Danish Natural Science Research Council (Grant 272-06-0566, SKAGIS), the National Natural Science Foundation of China (Grant 40676027), the Ministry of Science and Technology of China (Grant SKLEC-2008KYYW02) and the PhD Program Scholarship Fund of East China

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