Coherent deep flow variation in the Iceland and American basins during the last interglacial

doi:10.1016/S0012-821X(98)00209-X

Earth and Planetary Science Letters

Volume 164, Issues 1–2, 15 December 1998, Pages 15-21

https://doi.org/10.1016/S0012-821X(98)00209-X Get rights and content

Abstract

The first direct evidence that the deep flow speed of North Atlantic Deep Water (NADW) in the South Iceland and North American Basins declined dramatically over a period of a few hundred years at about 120,000 years ago is presented. The flow rate inferred from silt grain-size then rose into the subsequent cool period (stage 5d). These changes are clear and synchronous in records from both locations. These circulation events are recorded less markedly in chemical and isotopic proxies for the nutrient status of water masses. The congruence of flow variation and hydrography at the two separated sites suggests they are primarily controlled by variation in the deep thermohaline circulation.

Introduction

Heat flux to the northeastern North Atlantic during an interglacial is of key significance for the habitability of northwest Europe. Vigorous heat supply to high latitudes by surface ocean flow is accompanied by equally vigorous southward deep flow of this water after cooling and sinking in the Norwegian–Greenland Sea [1]. The peak of the last interglacial, marine oxygen isotope 5e and the Eemian interglacial on land have been the focus of much recent climate research on terrestrial and marine records. Climate shifts during this period are of major interest because they may give us clues to the initiation, development and demise of interglacial climate.

Two cores from different North Atlantic basins, but both under the influence of NADW flow, one close to its origin and the other down-stream in its evolution are examined here (Fig. 1). The first is from the northern flank of Charlie-Gibbs Fracture Zone (CGFZ) (NEAP-18K, 52°46.02′N, 30°20.68′W, 6.64 m long, 3275 m water depth). This is the southern extremity of Gardar Drift, which was deposited from the Deep Northern Boundary Current [5]of the Atlantic in the Iceland Basin [6]. The core has a near continuous record of sedimentation since the peak of the last interglaciation. Core MD95-2036 was taken from eastern Bermuda Rise (33°41.44′N, 57°34.55′W, 4462 m water depth), where the sediments are also deposited under current influence 7, 8. The 52.7 m core includes the last interglacial and preceding glacial (isotope stages 5e and 6) [9].

The NEAP-18K site lies presently under the path of Iceland–Scotland Overflow Water (ISOW) and Lower Deep Water (LDW) flow leaving the Iceland Basin 2, 3(Fig. 1). Approximately 6.6 sverdrups of mixed ISOW and LDW flow east to west through the CGFZ [1], contributing a significant fraction of the subsequent NADW mixture. Core MD95-2036 is also sited under NADW at present, with Southern Source Water (SSW, mainly Antarctic Bottom Water) at greater depths over the adjacent Sohm Abyssal Plain (5500 m depth). The flow over the Bermuda Rise is boosted by the presence of a recirculating gyre [2]. Because bottom currents exert primary control on sediment deposition and there is negligible interference by direct fall-out from icebergs at either site during the interglacial, they are optimal locations to record bottom water flow changes.

Section snippets

Methods

Here we employ an `intensive' physical proxy of relative current speed, the mean size of the 10–63 μm non-cohesive silt fraction (`sortable silt', hereafter $SS$ ) 10, 11, for which stronger currents, through both selective deposition and winnowing, yield a coarser mean size. The grainsize distribution of the fine terrigenous fractions was determined using a Sedigraph 5100. The $SS$ proxy provides an indication of relative current speed but is presently uncalibrated. Some idea of actual current

Stratigraphy

Ages for MD95-2036 are reported by Adkins et al. [9]and are based on δ¹⁸O stratigraphy and ²³⁰Th-constrained flux estimates. Ages for NEAP-18K have been estimated by correlating the benthic δ¹⁸O record with MD95-2036 at the mid-point in the substage 5e/5d transition (NEAP-18K 574 cm; MD95-2036 4307 cm = 117 ka BP) and the top of the section (496 cm) with event 5.33 (103.3 ka BP) in the SPECMAP stratigraphy [17](Fig. 2Fig. 3). The benthic δ¹⁸O in core NEAP-18K suggest that Termination II (TII)

Conclusions

Covariation throughout the $SS$ records from the South Iceland and North American Basins influenced by NADW thermohaline flow (and in the latter setting also by a recirculating gyre) suggests that the inferred vigour of palaeocirculation on the Bermuda Rise was primarily a response to changes in the flux of NADW rather than eddy-driven currents. Although abyssal eddy kinetic energy may ultimately be related to atmospheric (wind stress) forcing, it is not self-evidently in phase with and of the

Acknowledgements

We thank Edward Boyle for providing samples of IMAGES core MD95-2036. He and two anonymous referees contributed thoughtful reviews. We are extremely grateful to the officers, crew and other scientists on board Charles Darwin for their help during Cruise-CD88. This work was supported by UK NERC for NEAPACC. [MK]

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Coherent deep flow variation in the Iceland and American basins during the last interglacial

Abstract

Introduction

Section snippets

Methods

Stratigraphy

Conclusions

Acknowledgements

Prog. Oceanogr.

Prog. Oceanogr.

Quat. Res.

Mar. Geol.

Earth Planet. Sci. Lett.

The production of North Atlantic Deep Water: Sources, rates, and pathways

J. Geophys. Res.

On the North Atlantic circulation

Rev. Geophys.

Abyssal eddy kinetic energy in the North Atlantic

J. Mar. Res.

, Erosion and deposition on the eastern margin of the Bermuda Rise in the late Quaternary

Deep Sea Res.

Geological effects of the Gulf Stream system on the northern Bermuda Rise

Mar. Geol.

Variability of the North Atlantic thermohaline circulation during the last interglacial period

Nature