The Disko Bugt area in central West Greenland has attracted considerable research attention in recent years, and there have been studies of both oceanic variability [1
] and fluctuations in tidewater-based ice streams such as Jakobshavns Isbrae [5
]. In addition, special attention has been paid to the variation of the ice sheet and the deglaciation during the late Quaternary, and to the nearshore-to-offshore ocean circulation during the Holocene. The hydrography of the coastal waters of West Greenland is linked to the large-scale North Atlantic circulation system via the combined effects of the East Greenland Current (EGC) and the Irminger Current (IC), which merge to form the West Greenland Current (WGC; Figure 1
). Thus, summer sea-surface temperatures (SSTs) off West Greenland are largely determined by the relative flow of Polar and Atlantic water masses within the WGC. Multi-proxy reconstructions of SST along West Greenland have so far mainly been qualitative [2
]. However, quantitative SST reconstructions on a long timescale are necessary to help understand the dynamics of the ocean-climate system.
Various proxies have been used to reconstruct palaeoclimatic and palaeoceanographic changes in the northern North Atlantic [12
]. Among these, diatoms are a common and abundant component of Arctic phytoplankton communities, and their distribution is strongly influenced by surface water temperature and sea ice cover [25
]. In the North Atlantic region, diatoms have been used as a tool for quantitative reconstructions of summer SST and sea ice concentration (SIC) during the Holocene using the transfer function method [22
]. However, except for the reconstructions of SIC by Sha et al. [17
] and of SST and SIC by Krawczyk et al. [31
], there is a dearth of diatom-based reconstructions from West Greenland.
The Sun is the main driver of Earth’s climate [32
], and several studies have demonstrated that the effect of solar changes on regional modes of atmospheric variability is evident in the instrumental record [33
]. Solar activity has been proposed to influence variability in various components of the climate system, such as temperature, winds, precipitation, and drift ice [15
]. For example, Bond et al. [41
] found that Holocene drift ice in the North Atlantic and records of the cosmogenic nuclides 10
Be and 14
C are correlated, and concluded that variations in solar activity may have played a significant role in iceberg production and drift. Holzhauser et al. [42
] found that glacier advances and lake level changes are correlative with increased 14
C production rate (reduced solar activity). Furthermore, changes in SST reconstructions also exhibit very similar patterns to variations in solar activity. The strong correlation of the Norwegian Sea temperature with solar activity over the last millennium indicates that variations in solar activity affect regional atmospheric variability, which in turn controls the poleward transport and temperature of warm Atlantic surface waters [43
Here, we present a diatom-based summer SST record from a marine sediment core (gravity core DA06-139G) from Vaigat Strait, Disko Bugt, central West Greenland. Our objectives are twofold: firstly, to quantitatively reconstruct summer SSTs over the past 5000 years in Vaigat Strait; and secondly, to assess potential forcing mechanisms of summer SST variability.
2. Oceanographic Setting
Disko Bugt is a large marine embayment (68°30′–69°15′N, 50°00′–54°00′W; Figure 1
) in central West Greenland. Water depths generally range from 200 to 400 m, but reach 990 m in the deep submarine valley Egedesminde Dyb, which extends in a south-westerly direction [44
Vaigat Strait is situated to the north of Disko Bugt (Figure 1
B), and is bound by Disko Island to the southwest and by the Greenland mainland to the northeast. It acts as a major exit pathway for the WGC waters entering Disko Bugt, as well as for meltwater and icebergs generated by the major tidewater glaciers, including the Jacobshavn Isbræ, northwards into Baffin Bay [44
]. The strait is ca. 130 km long and 20–25 km wide, and has a maximum water depth of ~600 m [45
]. It is suggested that the passage acted as a northern conduit of an ice lobe extending to the mouth of the strait during the last glaciation [46
The surface circulation to the west of Greenland and adjacent areas is dominated by two major currents: the WGC, which flows northwards along the west coast of Greenland, and the Baffin–Labrador Current (BLC) which flows southwards along the east coast of Baffin Island and Labrador (Figure 1
A). The WGC consists of a combination of (i) Arctic-sourced cold, low-salinity water from the EGC (at 0–200 m water depth), designated Polar water [47
]; and (ii) relatively warm and saline Atlantic-sourced water from the IC (>200 m water depth), a branch of the North Atlantic Current (NAC, [47
]). The two components mix continuously as the WGC flows northwards, but can still be distinguished to the southwest of Disko Bugt [49
The reconstructed summer SSTs ranged from 1.4 to 5 °C, with a mean of 3.1 °C; in addition, a long-term cooling trend of the water masses in Vaigat Strait is evident during the past 5000 years (Figure 4
A). The summer SSTs were generally higher than the mean value before 3000 cal. a BP, except for short intervals of somewhat low summer SST between 4800 and 5000 cal. a BP. Subsequently, there was a gradual decrease in the reconstructed summer SST after 3000 cal. a BP which continued until about 2000 cal. a BP. A warm interval, centered at about 1700 cal. a BP was succeeded by decreasing SSTs, which reached their lowest values at about 1500 cal. a BP. There was a gradual increase in reconstructed summer SST during the interval from 1200 to 630 cal. a BP, followed by a distinct decrease after 630 cal. a BP. The prediction error for each sample ranges between 0.278 and 1.727 °C.
Spectral analysis of the record reveals statistically significant (above the 90% confidence level), centennial-scale periodicities centered at 529, 410, 191, 131, 124, and 114 years (Figure 5
). The 529, 410, and 191-year periodicities are close to the ubiquitous 512 and 206-year 14
C cycles [63
], which indicates that solar modulation may play a role in driving centennial-scale variations in SST in Vaigat Strait. However, the periodicities of ~114–131 years should be regarded as uncertain, because they are too close to the Nyquist frequency.
Modern diatom assemblages from 80 surface sediment samples from west of Greenland and around Iceland were used to establish a new diatom-based transfer function for reconstructing summer sea-surface temperature (SST). A gradual decrease in summer SSTs recorded in core DA06-139G reflects a cooling trend, with several fluctuations superimposed, during the last 5000 years in Vaigat Strait in Disko Bugt.
The reconstructed summer SST record ranges from 1.4 to 5 °C, with a mean of 3.1 °C. Summer SSTs were generally higher than the mean before 3000 cal. a BP, which is correlative with the Late-Holocene Thermal Maximum. There followed a step-by-step decrease in summer SSTs between 3000 and 2000 cal. a BP, known as the “Neoglacial cooling”. After 2000 cal. a BP, two cooling events at 1500–1200 cal. a BP and 630–50 cal. a BP are distinguished in the summer SST record, as well as two significantly warm episodes from 2000 to 1600 cal. a BP and from 1200 to 630 cal. a BP. Comparison of the summer SST record from Vaigat Strait with reconstructed temperatures at Kangerlussuaq lake, West Greenland, and the GISP2 ice core reveals coherent changes and a consistent pattern of variation, reflecting regional changes in SST in West Greenland.
Spectral analyses indicate that significant centennial-scale variations are superimposed on the long-term orbital trend. The dominant periodicities are 529, 410, and 191 years, which may be linked to the well-known 512- and 206-year solar cycles. Cross-correlation analyses between the summer SSTs and total solar irradiance through the last 5000 years indicate that the records are in phase, providing evidence that variations in solar activity impacted regional summer SST variability. Overall, the strong linkage between solar variability and summer SSTs is not only of regional significance, but is also consistent over the entire North Atlantic region.