Tritium and 3He in the Sargasso Sea

The systematics of tritium (3H), 3He and 3H-3He dating are investigated for oceanic mixing systems responding to the North Atlantic surface water tritium transient. Although the 3H-3He age is a single valued function of the true mixing age, verbatim acceptance of the 3H-3He age will result in a substantial underestimate of the mixing age for systems with timescales approaching the time elapsed since the tritium transient (1964-1965) or greater...

On the climate of a subtropical ocean gyre: Decade timescale variations in water mass renewal in the Sargasso Sea

A simple concept model of the climatology of a subtropical ocean gyre is developed on the basis of assumed isopycnal transport and latent heat flux. The 27 year record of annually averaged salinities on isopycnals near Bermuda shows clear and systematic variations, and significant (\u3e 95% confidence) correlations with oxygen (positive) and vertical density gradient (negative) support the hypothesis...

Rates of vertical mixing, gas exchange and new production: Estimates from seasonal gas cycles in the upper ocean near Bermuda

Argon measurements, obtained from one year of monthly detailed vertical profiles near Bermuda (32N 64W), show a maximum in argon supersaturation of about 4% in the seasonal thermocline in late summer. Since the argon supersaturation is 3–4 times smaller than that of oxygen, most of the oxygen supersaturation is not of physical origin and hence must result from biological production. In the winter mixed layer, air injection produces argon supersaturation in spite of high gas exchange rates. During spring and summer, radiative heating, air injection, and an upward argon flux create an even larger supersaturation in the mixed layer. In the seasonal thermocline, radiative heating maintains argon concentrations above solubility equilibrium in spite of vertical mixing. The observed seasonal cycles of temperature, argon, helium, and oxygen are simulated with an upper ocean model. We linearized the model\u27s response to variations in vertical diffusivity, air injection, gas exchange rate, and new production and then used an inverse technique (singular value decomposition) to determine the values of these parameters that best fit the data. A vertical turbulent diffusivity of 0.9 ± 0.1 × 10–4 m2 s–1 is consistent with both the thermal history and subsurface argon distribution. The rate of air injection, determined to ±25%, is similar to previous estimates. The seasonally-averaged gas exchange rate is 17 ± 12% lower than predicted by Liss and Merlivat (1986). We estimate a lower limit to depth-integrated new production below the mixed layer of 4.3 ± 1.7 moles O2 m–2 yr–1 during 1985, and obtain an estimate of 5.6 ± 1.5 moles O2 m–1 yr–1 if new production in the mixed layer is fixed at zero

The five stable noble gases are sensitive unambiguous tracers of glacial meltwater

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 2835–2841, doi:10.1002/2013GL058804.The five inert noble gases—He, Ne, Ar, Kr, and Xe—exhibit a unique dissolved gas saturation pattern resulting from the formation and addition of glacial meltwater to seawater. He and Ne become oversaturated, and Ar, Kr, and Xe become undersaturated to varying percentages. For example, addition of 10‰ glacial meltwater to seawater results in a saturation anomaly of ΔHe = 12.8%, ΔNe = 8.9%, ΔAr = −0.5%, ΔKr = −2.2%, and ΔXe = −3.3%. This pattern in noble gas saturation reflects a unique meltwater signature that is distinct from the other major physical processes that modify the gas concentration and saturation, namely, seasonal changes in temperature at the ocean surface and bubble mediated gas exchange. We use Optimum Multiparameter analysis to illustrate how all five noble gases can help distinguish glacial meltwater from wind-driven bubble injection, making them a potentially valuable suite of tracers for glacial melt and its concentration in the deep waters of the world ocean.We are grateful to the National Science Foundation (OCE825394 and OCE0752980) for support of this research.2014-10-1

Evolution of the south Pacific helium plume over the past three decades

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 18 (2017): 1810–1823, doi:10.1002/2017GC006848.The recent GEOTRACES Eastern Pacific Zonal Transect in 2013 crossed the East Pacific Rise at 15°S following the same track as the 1987 Helios Expedition along the core of the mid-depth helium plume that spreads westward from the East Pacific Rise (EPR) axis. The fact that several stations were co-located with the earlier Helios stations has allowed a detailed comparison of the changes in the helium plume over the intervening 26 years. While the plume in many areas is unchanged, there is a marked decrease in plume intensity at longitude 120°W in the 2013 data which was not present in 1987. Recent radioisotope measurements along the plume track suggest that this decrease is due to the intrusion of a different water mass into the plume, rather than a modulation of hydrothermal input on the EPR axis. Analysis of GEOTRACES hydrographic data shows excess heat present in the plume up to 0.04°C, corresponding to a 3He/heat ratio of ∼2.5 × 10−18 mol J−1, similar to that found in mature hydrothermal vents. RAFOS floats deployed in 1987 indicate an average westward transport of ∼0.3 cm s−1 at 2500 m depth in the off-axis plume, in agreement with recent estimates of ∼0.4 cm s−1 based on “aging” of the plume from 227Ac/3He ratios.Earth Ocean Interactions Program; NOAA Pacific Marine Environmental Laboratory2017-11-0

Observations of temporal changes of tritium-3He age in the eastern North Atlantic thermocline: Evidence for changes in ventilation

A compilation of fifteen years of tritium and 3He measurements is used to examine the ventilation of the eastern North Atlantic subtropical gyre with specific emphasis on the temporal character of the tracer age field. A multivariate regression analysis in the form of a spatiotemporal Taylor expansion is applied to observations interpolated onto isopycnal surfaces. The time-dependent component of the tracer age field is found to be statistically significant, explaining approximately 10% of the variance of the tracer age observations in the upper thermocline (σ = 26.5) and increasing to roughly 50% of the variance in the lower thermocline (σ = 27.0). The observed transient tracer age increases over the 15 years of observations with the fractional rate of change of the age field varying between 2% and 5% per year. The largest observed changes occur on the deepest, most slowly ventilated isopycnal surfaces. The second derivative of the tritium-3He age with time suggests that the tracer age field may be approaching a steady state. If tritium-3He age is interpreted as a true measure of the advective ventilation age, the temporal changes in age would imply a slackening of the ventilation of the lower main thermocline of greater than 50% from the late 1970\u27s to the early 1990\u27s. However, consideration of the full advective-diffusive balance of tritium-3He age reveals that the changes in tracer age field represent a time-dependent adjustment of the transient tracer concentrations in conjunction with a steady local circulation field. Integral approximations of the upstream evolution of the tracer field also fail to demonstrate evidence for decadal changes in ventilation. The integral balance along the path of subduction yields an improved estimate of the true ventilation age based on the temporal tendency of the age field along the path of ventilation. An approximation of this integral suggests that actual ventilation ages can be up to 40% larger than the measured tracer age in the deeper portions of the North Atlantic thermocline. Proper accounting of the time-dependent biases of the tracer age dating technique are a prerequisite for examining transient tracer measurements for evidence of changes in the physical ventilation of the upper ocean

The effect of boundary conditions on tracer estimates of thermocline ventilation rates

Using a simple box mixing model, we show that ventilation rate estimates obtained from tracer box models may be significantly smaller than fluid replacement rates. The degree to which a tracer ventilation estimate approaches the actual (fluid) ventilation rate depends on the surface boundary condition for that tracer. Ventilation rates for rapidly exchanging tracers (e.g. 3He) are close to the fluid ventilation rate while tracers with limited surface exchange (e.g. tritium) ventilate more slowly. For box mixing models, the ratio of ventilation rates for limited surface exchange tracers to rapidly exchanging tracers approaches the ratio of summer to winter mixed layer depths. Further, the distribution of rapidly equilibrating tracers more accurately tracks climatological fluctuations in water mass formation rates. Limited surface exchange tracers show a damping proportional to the ratio of summer to winter mixed layer depths. To compare model results with observations, we calculate 3He and tritium ventilation rates from data taken in 1979 in the eastern subtropical North Atlantic. In calculating the tritium ventilation rates, we modify a North Atlantic tritium “source function” (time history of-surface water tritium concentrations), extending previous work using recent data. On shallow density surfaces (σ \u3c 27.0), the computed tritium ventilation rates are 2–3 times slower than those for 3He, in agreement with climatological ratio of summer to winter mixed layer depths. Deeper in the thermocline, the two tracer estimated ventilation rates converge. This trend may be related to the decreasing effectiveness of 3He gas exchange in equilibrating the deeper winter mixed layers of the more northerly isopycnal outcrops. We conclude that box models using limited surface exchange tracers (e.g. 14C and tritium) can under predict oxygen utilization rates (OUR) by up to 3 times due to differences in tracer boundary conditions, while a tracer like 3He may overestimate OUR by 10–20%

Spreading of Greenland meltwaters in the ocean revealed by noble gases

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7705–7713, doi:10.1002/2015GL065003.We present the first noble gas observations in a proglacial fjord in Greenland, providing an unprecedented view of surface and submarine melt pathways into the ocean. Using Optimum Multiparameter Analysis, noble gas concentrations remove large uncertainties inherent in previous studies of meltwater in Greenland fjords. We find glacially modified waters with submarine melt concentrations up to 0.66 ± 0.09% and runoff 3.9 ± 0.29%. Radiogenic enrichment of Helium enables identification of ice sheet near-bed melt (0.48 ± 0.08%). We identify distinct regions of meltwater export reflecting heterogeneous melt processes: a surface layer of both runoff and submarine melt and an intermediate layer composed primarily of submarine melt. Intermediate ocean waters carry the majority of heat to the fjords' glaciers, and warmer deep waters are isolated from the ice edge. The average entrainment ratio implies that ocean water masses are upwelled at a rate 30 times the combined glacial meltwater volume flux.We gratefully acknowledge funding from WHOI's Ocean and Climate Change Institute, the Doherty Postdoctoral Scholarship, and ship time from the Advanced Climate Dynamics Summer School (SiU grant NNA-2012/10151).2016-03-3

Export of strongly diluted Greenland meltwater from a major glacial fjord

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 4163-4170, doi:10.1029/2018GL077000.The Greenland Ice Sheet has been, and will continue, losing mass at an accelerating rate. The influence of this anomalous meltwater discharge on the regional and large‐scale ocean could be considerable but remains poorly understood. This uncertainty is in part a consequence of challenges in observing water mass transformation and meltwater spreading in coastal Greenland. Here we use tracer observations that enable unprecedented quantification of the export, mixing, and vertical distribution of meltwaters leaving one of Greenland's major glacial fjords. We find that the primarily subsurface meltwater input results in the upwelling of the deep fjord waters and an export of a meltwater/deepwater mixture that is 30 times larger than the initial meltwater release. Using these tracer data, the vertical structure of Greenland's summer meltwater export is defined for the first time showing that half the meltwater export occurs below 65 m.National Science Foundation Grant Number: OCE-15368562018-11-0

Characteristics of meltwater export from Jakobshavn Isbræ and Ilulissat Icefjord

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Annals of Glaciology 58 (2017): 107-117, doi:10.1017/aog.2017.19.Jakobshavn Isbræ, which terminates in Ilulissat Icefjord, has undergone rapid retreat and is currently the largest contributor to ice-sheet mass loss among Greenland’s marine terminating glaciers. Accelerating mass loss is increasing fresh water discharge to the ocean, which can feed back on ice melt, impact marine ecosystems and potentially modify regional and larger scale ocean circulation. Here we present hydrographic observations, including inert geochemical tracers, that allow the first quantitative description of the glacially-modified waters exported from the Jakobshavn/Icefjord system. Observations within the fjord suggest a deep-reaching overturning cell driven by glacial buoyancy forcing. Modified waters containing submarine meltwater (up to 2.5 ± 0.12%), subglacial discharge (up to 6 ± 0.37%) and large portions of entrained ocean waters are seen to exit the fjord and flow north. The exported meltwaters form a buoyant coastal gravity current reaching to 100 m depth and extending 10 km offshore.We gratefully acknowledge support from WHOI’s Ocean and Climate Change Institute, the WHOI Doherty Postdoctoral Scholarship, the US National Science Foundation grant NSF OCE-1536856, and the leaders and participants of the Advanced Climate Dynamics Summer School (SiU grant NNA-2012/10151). Ship-based CTD data are freely available from the NOAA National Centers for Environmental Information, discoverable with Accession Number 0162649. Expendable CTD data are included in the Supplementary Material