Abyssal origin for the early Holocene pulse of unradiogenic neodymium isotopes in Atlantic seawater

Jacob N.W Howe,Alexander M Piotrowski,Victoria C.F Rennie

doi:10.1130/g38155.1

Abstract

The neodymium isotopic composition of authigenic phases of deep-sea sediment cores can be interpreted as reflecting past changes in water-mass mixing proportions if end-member water-mass compositions are constrained through time. Here we present three new records spanning 2480 to 4360 m depth in the North Atlantic Ocean that show seawater Nd isotope values in the early to mid-Holocene that are more radiogenic than values from the abyssal northwest Atlantic. This finding indicates that that the end-member composition of North Atlantic Deep Water was more stable within its core than it was at abyssal depths. The spatial distribution of the unradiogenic neodymium isotope values observed in the North Atlantic suggests a bottom source, and therefore that they were unlikely to have been due to the production of intermediate-depth Labrador Sea Water. We infer that the unradiogenic authigenic Nd isotope values were most likely derived from a pulse of poorly chemically weathered detrital material that was deposited into the Labrador Sea following Laurentide ice sheet retreat in the early Holocene. This unradiogenic sediment released neodymium into the bottom waters, yielding an unradiogenic seawater signal that was advected southward at abyssal depths and attenuated as it vertically mixed upward in the water column to shallower depths. The southward dispersion of these unradiogenic seawater values traces deep-water advection. However, the exact values observed at the most abyssal sites cannot be interpreted as proportionate to the strength of deep-water production without improved constraints on end-member changes.

Highlights

Changes in North Atlantic Deep Water (NADW) production, inferred from paleoceanographic proxy evidence, are thought to be integral to past changes in the climate (Roberts et al, 2010)
In this work we present foraminiferal eNd records of the past 23 k.y. from 3 sites that span from 2480 to 4360 m depth in the Atlantic (Fig. 1), to provide new constraints on the spatial extent of the unradiogenic eNd values observed in the North Atlantic in the early Holocene (Gutjahr et al, 2008; Colin et al, 2010; Roberts et al, 2010; Crocket et al, 2011; van de Flierdt et al, 2006; Lippold et al, 2016)
The SU90-03 (2480 m) record is very similar to the authigenic eNd record from the deep Bermuda Rise (4540 m; Fig. 2B), except that its early Holocene peak is significantly more radiogenic than that of the deep Bermuda Rise site (Roberts et al, 2010)

Summary

Introduction

Changes in North Atlantic Deep Water (NADW) production, inferred from paleoceanographic proxy evidence, are thought to be integral to past changes in the climate (Roberts et al, 2010). The authigenic phases of seafloor sediment cores have been used to infer changes in seawater eNd, and water-mass mixing and NADW production, in the past (e.g., Gutjahr et al, 2008; Roberts et al, 2010; Crocket et al, 2011; Böhm et al, 2015; Lippold et al, 2016) This interpretation is complicated by studies based on the Bermuda Rise in the abyssal northwestern Atlantic that have found eNd values that are more negative than the modern composition of NADW during warm climate periods with values as low as -16.2 in the early to mid-Holocene (Roberts et al, 2010), -16.0 during the interstadials of Marine Isotope Stage (MIS) 3, The spatial extent of these unradiogenic neodymium isotope values is poorly constrained, especially at intermediate to mid-depths. Given that the southward flux of NADW is stronger at intermediate to mid-depths (1000–3000 m) than in the abyssal Atlantic (Kuhlbrodt et al, 2007), placing constraints on the vertical distribution of the eNd values in the North Atlantic during warm periods is essential to correctly determining the relationship of eNd to Atlantic overturning circulation

Methods

Results

Conclusion