Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction

Abstract

The isotopic composition of neodymium (εNd) extracted from sedimentary Fe-Mn oxyhydroxide offers potential for reconstructing paleo-circulation, but its application depends on extraction methodology and the mechanisms that relate authigenic εNd to bottom water. Here we test methods to extract authigenic εNd from Gulf of Alaska (GOA) sediments and assess sources of leachate Nd, including potential contamination from trace dispersed volcanic ash. We show that one dominant phase is extracted via leaching of core-top sediments. Major and trace element geochemistry demonstrate that this phase is authigenic Fe-Mn oxyhydroxide. Contamination of leachate (authigenic) Nd from detrital sources is insignificant (<1%); our empirical results are consistent with established kinetic mineral dissolution rates and theory. Contamination of extracted εNd from leaching of volcanic ash is below analytical uncertainty. However, the εNd of core-top leachates in the GOA is consistently more radiogenic than bottom water. We infer that authigenic phases record pore water εNd, and the relationships of εNd among bottom water, pore water, authigenic phases and detrital sediments are primarily governed by the exposure time of bottom water to sea-floor sediments, rate of exchange across the sediment-water interface and the reactivity and composition of detrital sediments. We show that this conceptual model is applicable on the Pacific basin scale and provides a new framework to understand the role of authigenic phases in both modern and paleo-applications including the use of authigenic εNd as a paleocirculation tracer.

The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

“…While this mechanism is clearly far from resolved, it offers an explanation for the apparent lack of influence from the non-radiogenic lithogenic sediments of the central North Pacific (Jones et al, 1994). Specifically, we predict that there is no positive benthic flux from these sediments, or that reactive diagenetic materials are not present in these sediments (Abbott et al, 2016;Du et al, 2016), or both. We also note that the nonradiogenic Nd data presented by Jones et al (1994) reflect the post-leach lithogenic fraction of the sediments, which may not play a role at all in defining the benthic flux as defined by our proposed mechanism.…”

“…In the benthic control model, such correspondence will result from bottom water that is either dominated by a strong positive benthic flux, wherein the preformed bottom water ε Nd signature is overwhelmed by a pore water signature supported by the authigenic phases, or from a negative or net-zero pore water flux, such that pore water and authigenic phases passively record bottom water signatures. There is also a range of intermediate conditions where ε Nd of the archival phase may differ from bottom water, and where we would predict there is a significant, but not overwhelming, positive benthic flux into bottom water (e.g., Du et al, 2016 in the Pacific, or Gutjahr et al, 2008;Huck et al, 2016 in the Atlantic). The relationship between authigenic phases (used as archival records) and contemporaneous bottom water is far more complex in the benthic control model, because the authigenic phases associated with diagenesis are intimately associated with the nature of the benthic flux that impact the bottom water signal (Abbott et al, 2016).…”

“…The concept of benthic control of [REE]/ε Nd on Pacific waters is still a nascent idea (Abbott et al, 2015a(Abbott et al, , 2016Du et al, 2016). Here, we expand on this idea in a general sense.…”

“…While such a correlation is not obligatory under the benthic control model, work with Pacific sediments suggests that certain components of the sediments may be more reactive during diagenesis, and such reactivity is also likely element specific (Abbott et al, 2016;Du et al, 2016). Thus, we might expect that reaction of these sedimentary elements imposes a covariation between the magnitude and isotopic composition of the flux.…”

“…Theory and observations all indicate that some aspect of bottom water ε Nd is maintained in authigenic phases, and is likely identifiable in all sediment (Gutjahr et al, 2007;Wilson et al, 2013;Blaser et al, 2016;Du et al, 2016). Furthermore, the additional Nd generated through diagenesis should be far less variable and readily constrained as "baseline" variations in sedimentary records, vs. higher-order changes that are derived from seawater; analogous to Sr isotope records.…”