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Geography and Environmental Science

Research project: Holocene Land-Ocean-Atmosphere Interactions on the Eastern Seaboard of North America (PRECIP)

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NERC-funded project, running 2009-2013. PRECIP is a NERC-funded collaboration between the universities of Southampton, Aberdeen Swansea and Exeter, led by Dr. Paul Hughes. The overall aim of the PRECIP project is to exploit the close coupling between raised bogs and the atmosphere to reconstruct the spatial-temporal pattern of moisture balance changes associated with past variability in the Gulf Stream and Labrador Current on the eastern seaboard of North America. This Holocene reconstruction will be generated at (multi-) decadal to millennial time-scales and it will be used to test a set of hypotheses concerning the relationships between ocean thermohaline variability, atmospheric circulation, and terrestrially-based palaeoclimate responses.

Land-based environmental responses to climate-ocean variability in the North Atlantic region are a critical research priority because they show how changes in key climate drivers such as North Atlantic Deep Water (NADW) formation and the Atlantic Meridional Overturning Circulation (AMOC) may be transmitted to environmental phenomena of socio-economic importance such as the atmospheric water balance and soil moisture availability. Past records show a persistent pattern of centennial-to-millennial scale changes in ocean circulation during the Holocene (last 11,700 years) in the mid- and eastern North Atlantic Ocean that has been linked to ice-rafting. Likewise, in the western Atlantic significant variability in sea surface temperatures (SST) and salinity has been deduced for the period before 6.8ka, driven by discharges from the Laurentide Ice Sheet. Given the past sensitivity of ocean circulation to freshwater inputs from melting ice sheets, modern observations of increased discharge from Arctic rivers and accelerating ablation rates in Greenland have raised concern over the potential for significant reorganisations of oceanic heat transport under future climate scenarios, leading to major impacts on terrestrial ecosystems. Whilst we are beginning to understand the nature and magnitude of Holocene changes in the circulation of the N. Atlantic Ocean, there is a poor understanding of terrestrially-based responses to these events in terms of magnitude and spatial variability as well as driving mechanisms. The PRECIP project aims to address these deficiencies.

Figure 1: Location of places referenced in the text and the transect of study sites (position of the Gulf Stream and Labrador Current after Keigwin et al., 2005).

Figure 1

The so-called ‘8.2ka cooling event’ is the best known example of a period of variability of ocean circulation and climate occurring during the Holocene, forced by a megaflood from the former Laurentide Ice Sheet (North American Ice Sheet). Model simulations of the ‘8.2 ka event’ suggest that greatly reduced North Atlantic Deep Water (NADW) formation occurred together with a southward shift of the western NADW formation area from 60° N to 45° N. Modelling suggests that relatively small meltwater injections into the Labrador Sea might be expected to produce weakening and latitudinal migration of the site of NADW formation. Land areas adjacent to the meltwater transit routes should also show a strong response to meltwater and ice-rafting events. The PRECIP project is exploring the expression of these events on land.

Following the end of major meltwater discharges from the Laurentide Ice Sheet at 6.8 ka SST and salinity records stabilised in the Labrador Sea and western Atlantic. Some researchers have suggested that the subsequent climate of this region was relatively complacent, in contrast to the continuing centennial-to-millennial-scale variability of the mid- and eastern Atlantic, associated with pronounced changes in drift ice and deep convection. However, terrestrial palaeoclimate records from Newfoundland and continental N. America suggest that centennial-to-millennial-scale variability in the inferred atmospheric moisture balance persisted throughout the mid- to late Holocene, albeit with a lower magnitude than events predating 6.8ka. The PRECIP project will test these drivers of centennial-to-millennial-scale change in the atmospheric moisture balance of eastern N. America by deriving terrestrial reconstructions of past atmospheric moisture balance (P-E) and “water isotopes” of oxygen and hydrogen (δ18O and δD) in precipitation from sites along a N-S gradient on the eastern seaboard of N America and by comparing them with a range of existing palaeoenvironmental records.

The raised bogs of eastern Newfoundland, Nova Scotia and Maine are ideally placed to register the onset and termination of Holocene meltwater and ice-rafting events from the Hudson Bay and Ungava areas. Raised bog hydrology is particularly sensitive to the length of the summer water deficit and therefore peat palaeohydrological records provide a responsive record of past changes in summer surface moisture conditions in this region.

Figure 2: 8700 year-long peat-based palaeoclimate reconstruction from Newfoundland using oxygen isotope data from Sphagnum leaf cellulose (A) and palaeo-hydrological proxy data (B) compared with GRIP δ18O ice record on the GICC05 timescale (C).

(A) Estimated δ18Oprecipitation variations using the constant isotopic fractionation factor (1.0274) between precipitation and Sphagnum cellulose (Daley, 2007).

(B) Composite record of changes in bog water table using three bog proxies, expressed as deviations from the long-term average. Positive values = dry conditions Hughes et al. 2006

(C) δ18Oice data from the Greenland (GRIP) ice core (Rasmussen et al., 2006; Vinther et al. 2006).

Figure 2

In summary, the overall aim of the PRECIP project is to exploit the close coupling between raised bogs and the atmosphere to reconstruct the spatial-temporal pattern of moisture balance changes associated with past variability in the Gulf Stream and Labrador Current on the eastern seaboard of North America at (multi-)decadal to millennial time scales. This reconstruction will be used to test a set of hypotheses concerning the relationship between THC variability, atmospheric circulation, and terrestrial climate responses.

Sampling a plateau bog in northern Newfoundland
Paul Hughes
on Burnt Village Bog, NFL
Sampling vegitation
at Petote Bog, NS
Surveying stratigraphy cores
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