Radiogenic isotope tracers of present and past ocean circulation
Dr Tina van der Flierdt, Imperial College London, UK
The deep ocean contains ~60 times more carbon than the atmosphere and a large part of the carbon exchange takes place in areas of deep water formation and upwelling. Therefore small changes in the rate of deep water formation are likely to have a large impact on the atmospheric carbon budget. Hence it is – and has been – of great interest to decipher the relationship between climate change and ocean circulation. While we have ample ways to look at ocean circulation in the modern ocean, we struggle to reconstruct physical oceanography in the past. Neodymium (Nd) isotopes have been considered a promising ‘quasi-conservative’ water mass tracer for many years, and have been used extensively to reconstruct ocean circulation, on million year, glacial-interglacial, and even more rapid (sub)millennial timescales.
While published Nd isotope records show intriguing co-variations with other climate indicators, our understanding of the modern cycle of Nd in the ocean has been rather poor. A good understanding of modern processes is, however, critical before a palaeo tracer can be applied with confidence to study the past. In this talk I will assess where we stand on using Nd isotopes as a palaeo-circulation tracer by scrutinising the wealth of new data collected on modern seawater in the context of the international GEOTRACES programme, which more than doubled the observational database over the past five years. Evaluating this data set reveals critical and novel information to be considered for palaeo interpretations, with implications on the ocean’s role in climate change.
Transition metal isotopes as tracers of oceanic metal budgets and cycling
Professor Derek Vance, ETH Zürich, Switzerland
The GEOTRACES programme has come at a propitious time, when we are newly able to study trace metal isotope variations in seawater. This contribution will review the progress of this new endeavour, with an emphasis on two distinct but related applications:
(1) One of the most dramatic features of ocean chemistry is the extreme transfer of metals from the surface to the deep associated with nutrient-like behaviour. For some metals this internal cycling is associated with isotope fractionation. But for others even near complete uptake in the photic zone does not come with a significant isotope effect. An understanding of this dichotomy can only emerge from a more active pursuit of process studies that focus on phytoplankton uptake mechanisms and their controls, including the interplay between different trace metals. We suggest that some oceanic hubs, in particular the high-latitude oceans, are proving to be just as key to trace metals as they are for major nutrient distributions.
(2) Isotope systems can help us to quantify and understand whole ocean transition metal budgets, including inputs and outputs, and the processes that control them. In a larger-scale perspective an important emerging theme concerns the contrasting isotopic impact of open ocean, oxic, versus marginal, anoxic/sulphidic, sinks from the dissolved pool to sediment. The contrast has implications for understanding ocean chemistry in deep time, and may turn out to be one of the more important applications of newly-developing transition metal isotope systems in earth science.
U-series rate-meters for ocean processes
Professor Gideon Henderson FRS, University of Oxford, UK
The distribution of trace-elements in the ocean is controlled by their input, removal and internal biogeochemical cycling. Assessing the rates at which these processes move trace metals is fundamental to understanding the modern distribution, the supply of trace metals to photic surface waters, and the response of ocean biogeochemistry to change. The differential solubility of the isotopes in the U and Th decay chains, coupled to their wide range of half-lives, provide many tools with potential to quantify the rates of processes involved in trace-metal cycling.
Insoluble Th has recently been shown to be a tracer for dust input, combining long-lived 232Th with shorter-lived 230Th to quantify the rates of modern dust addition averaged over a few years. The 230Th removed from seawater accumulates in underlying sediment where it provides a proxy for the rate of sedimentation and of sediment dissolution, and is used to normalise sedimentary 231Pa concentration to provide a controversial proxy for the rates of past ocean circulation. The shorter lived 234Th can be used to assess the downward flux of trace elements due to biological producitivity.
Soluble isotopes of Ra also provide powerful rate information, particularly related to the rates of mixing and advection in the modern ocean. This talk will overview recent and new data illustrating the power of U-series isotopes to quantify the operation of ocean trace-element cycles.
Nitrogen isotope variations as tracers of marine nitrogen biogeochemistry
Dr Karen Casciotti, Stanford University, USA
Nitrogen (N) isotopes are a powerful tool for tracking N cycling in past and present marine environments. Many of the biogeochemical processes that comprise the marine N cycle lead to isotopic fractionation between pools of N, and the intensities of these activities are recorded in the isotope ratios of the substrates and products of the reactions. Denitrification, the microbial reduction of nitrate to nitrite and gaseous products, occurs in subsurface oxygen deficit zones (ODZs) and is a dominant process in the marine N budget. Its activity leads to removal of fixed (bioavailable) N, potentially lowering marine productivity on a variety of space and time scales. Denitrification also leads to enrichment of 15N in nitrate, and the effects of this process can be observed long distances from ODZs. These signals can be mixed and advected out of the ODZ, or may be upwelled to the surface and incorporated into particulate organic matter. This transfer of ODZ signals into phytoplankton biomass and preservation in sediments leads to historical records of the intensity of water column denitrification where N uptake in surface waters is complete. N isotope data in nitrate and nitrite from a series of cruises to the Eastern Tropical South Pacific (ETSP), including GEOTRACES GP16, are used to more closely investigate the pathways of N transformation in the ETSP, as well as the transmission and alteration of N isotope signals beyond the oxygen deficient waters.