The net rate of freshwater input to the Arctic Ocean has been calculated in the past by two methods: directly, as the sum of precipitation, evaporation and runoff, an approach hindered by sparsity of measurements, and by the ice and ocean budget method, where the net surface freshwater flux within a defined boundary is calculated from the rate of dilution of salinity, comparing ocean inflows with ice and ocean outflows. Here a third method is introduced, the geochemical method, as a modification of the budget method. A standard approach uses geochemical tracers (salinity, oxygen isotopes, inorganic nutrients) to compute “source fractions” that quantify a water parcel's constituent proportions of seawater, freshwater of meteoric origin, and either sea ice melt or brine (from the freezing-out of sea ice). The geochemical method combines the source fractions with the boundary velocity field of the budget method to quantify the net flux derived from each source. Here it is shown that the geochemical method generates an Arctic Ocean surface freshwater flux, which is also the meteoric source flux, of

The global climate is changing

We define a flux of freshwater to mean the rate of addition of pure water to (or its removal from) the ocean surface, by exchanges with the atmosphere (evaporation,

The second way to estimate

We here introduce a third method as a modification of the budget method, which we call the geochemical method, and which requires knowledge of distributions of certain tracers that describe various sources of ocean waters. These tracers can be used to generate source fractions, and we aim to combine those source fractions with the TB12 velocity field to calculate new estimates of source fluxes. We next describe the candidate tracers and their functions.

Bulk ocean waters display a near-constant ratio of oxygen isotope concentration, measured as the anomaly from the ocean standard value,

Concentrations of dissolved inorganic nutrients in seawater and the elemental composition of phytoplankton populations are observed to occur at broadly the same stoichiometric ratios

It is observed that Pacific seawater has higher relative concentrations of phosphate than Atlantic seawater

Our aims in this study are (1) to generate new estimates of Arctic Ocean source fluxes using the geochemical approach, (2) to compare the results of the established budget approach to those of the new geochemical approach, and (3) to test the consistency of the various tracers used. To these ends, we first describe the data sources and the model used along with the attribution methods and schemes implemented (Sect.

TB12 use an inverse model

Map of the Arctic Ocean, showing the four main gateways. The position of the

From the TB12 model, the Arctic boundary circulation is broadly conventional. Atlantic-origin seawater enters through the Barents Sea Opening with a volume flux of

Biogeochemical data were originally collated and published by

Our domain comprises a total of 147 hydrographic stations, which includes data from 16 general circulation model grid cells in the Barents Sea Opening that are used as hydrographic stations, covering a total oceanic distance of 1803

Sections of

The

Following established practice, the sources of a parcel of oceanic water are considered to number three or four. The sources are characterized by end members, which are defined points in the phase space populated by the observed liquid (and solid i.e. sea ice) biogeochemical tracer properties, so that here “oceanic water” means the sum total of all liquid fractions. The term seawater is used to mean the typical source water fraction from the Atlantic (and also Pacific) Ocean; seawater fractions are always positive. The “meteoric” fraction can in principle be either positive, stemming directly or indirectly from rain- and snowfall, where the indirect route implies river runoff or terrestrial glacial input to the ocean, or negative, from evaporation. The “ice-modified” fraction is a result of sea ice freezing and melting, and (as will become apparent) appears mainly in oceanic water as negative fractions consequent on the freezing out of sea ice from oceanic water. For simplicity, therefore, we define this (negative) fraction as “brine”, following

We employ three variants of the approach to the calculation of the resulting source fractions. Firstly a three-end-member scheme (3EM) is adopted, which uses salinity and

Description of the three model schemes.

To discriminate between Atlantic and Pacific seawaters, an additional relationship is formulated in terms of the concentrations of the inorganic nutrients phosphate and nitrate

To quantify source fractions for each oceanic water parcel (i.e. grid point), we establish the following system of equations. This problem is conventionally treated as “square”, with the number of constraints equal to the number of source water fractions to be determined for each water parcel. Each water parcel then has a suite of

Previous studies have used different values for the end-member concentrations of salinity,

End-member values for salinity and

P : N relationships, where

The relationships between salinity and

Considering the published nitrate–phosphate relationships, the most appropriate to this study are the values used by

We use the approach established by TB12 and developed by

Then in the stationary case the surface freshwater flux

Due to the wide range of plausible end-member values for each of the water types, to give an estimate of the likely uncertainty due to end-member choice, fluxes of the different water types were evaluated using a Monte Carlo technique. Distributions for the different end-member parameters were constructed from the cited values (Table

Parameter space for the Monte Carlo simulations. Solid red line indicates the mean of the published values for the parameter; dashed red lines indicate maximum and minimum of published values.

For each model approach, fluxes of the different water types were estimated by combining the velocities from the TB12 model with the calculated water type fractions for the sample ensemble. Mean and standard deviations for the attributed volume fluxes of each water type were calculated as the mean and standard deviation of the results from the sample ensemble.

Here we present the results of the application of the methods and end members, described in Sect.

The distribution of 3EM source fractions is shown in Fig.

Sections of ice-modified fraction

Sections of ice-modified water flux

Typical volume fluxes (positive indicating into the Arctic) for the 3EM source fractions are shown in Fig.

Mean volume fluxes (

For the 3EM model schemes, the net seawater volume flux is effectively zero (

Mean volume fluxes (

The 3EM model indicates that the volume export of meteoric water through Fram Strait is concentrated in the Belgica Bank and East Greenland Current regions,

The 4EM scheme extends the 3EM scheme through use of inorganic nutrient (nitrate and phosphate) data, aiming to discriminate between Atlantic and Pacific seawater origin. The 4EM scheme retains single end points for salinity and

Sections of ice-modified fraction

Sections of ice-modified water flux

Sections of ice-modified fraction

Sections of ice-modified water flux

Similar to the 3EM model, both 4EM and 4EM+ models allocate the bulk of the ice-modified waters, mainly brine with some meltwater input, to the surface/upper waters. However, both four-end-member schemes indicate small but non-zero fractions (

Mean volume fluxes (

Differences between the three- and four-end-member model schemes are also reflected in the fluxes of the different fractions. For both four-end-member models, there are non-zero fluxes of brine, meteoric water (both

Meteoric and ice water volume fluxes.

The net ice-modified water (mainly brine) flux for both the 4EM and 4EM+ schemes is also consistent with the 3EM model and the TB12 solid ice flux, with the 4EM model estimating

While the net volume flux of meteoric water for the 4EM model is the same as that of the 3EM (

Both 4EM and 4EM+ model schemes indicate an imbalance in the net volume fluxes for both Pacific and Atlantic water. They both show a net export of Pacific water (4EM

Mean volume fluxes (

Mean volume fluxes (

Mean volume fluxes (

For the Fram Strait, the pattern of water fluxes described by both the 4EM and 4EM+ schemes is consistent with the pattern described above for the 3EM model (Tables

The description of Arctic freshwater fluxes presented by the 4EM+ model is broadly consistent with that from previous studies of fluxes in the Fram Strait using 4EM+ type schemes with distinct Pacific seawater,

The greatest differences between the models are in the fluxes of meteoric, brine and ice meltwaters across Belgica Bank and in the East Greenland Current (Fig.

In the Davis Strait the 4EM+ model is qualitatively consistent with previous studies, where source fractions show the highest freshwater content in the surface waters on the western side of the strait, from Pacific seawater and meteoric fractions, with a contribution from brine

Within uncertainty, the net seawater flux of the 3EM and 4EM models is zero:

The models generate apparent brine imports in the West Spitsbergen Current and the Barents Sea Opening, both with magnitude of

Mean volume fluxes (

Mean volume fluxes (

A consistent interpretation of the apparent West Spitsbergen Current and Barents Sea Opening brine imports, therefore, is that they are actually manifestations not of local processes but rather of source water variability, in the light of our salinity (

A second point concerns the near-total absence of positive ice-modified fractions, representing sea ice melt, anywhere around the boundary (Fig.

Thirdly, we know that sea ice is frozen out of liquid seawater, and it leaves behind in the seawater a negative

The only change in the 4EM model over 3EM is the inclusion of

The distribution of the 4EM Pacific fraction around the Arctic Ocean boundary (Fig.

We acknowledge that much remains unknown about the Arctic Ocean biogeochemical cycle; understanding of denitrification is at an early stage, and understanding of Arctic Ocean sources and sinks of nitrate and phosphate is incomplete

Another inconsistency arises from consideration of results from the 4EM+ model (Tables

A primary positive result of this study is the finding that both variants of the 3EM model (and the 4EM model) robustly quantify the net rate of Arctic meteoric freshwater input (the net of

An inconsistency arises from consideration of the composition and “labelling” of the waters of Bering Strait. Water entering the Arctic through the Bering Strait should, by definition, be seawater of Pacific origin. However, the Bering Strait inflow is unusually fresh because it contains a significant fraction of meteoric freshwater

The results of using at least partially degenerate constraints on the model fluxes are most clearly manifested in the 4EM+ model. The models with single seawater end point values (3EM and 4EM) have near-zero net seawater export (actually

Our geochemical approach to oceanic water flux calculation employs three valid and geochemically distinct categories of water: sea ice (in its various manifestations), meteoric (surface-origin) freshwater and seawater (where seawater is the component of the mixture that contains all of the dissolved salts). First, we note again that our total sea ice flux, being the sum of the fluxes of solid sea ice, sea ice meltwater and the freshwater deficit (brine) in the seawater from which the ice was formed, is approximately zero. Second, the TB12 velocity field is constrained to conserve salinity, and this is reflected in our zero net seawater fluxes, which is another statement of salinity conservation because seawater is the category that contains all of the ocean salinity. Third, we note that the same categories (both here and in TB12) of surface-origin freshwater are all meteoric, as the net of

We find the category Pacific water, defined from the N : P ratio, to be non-conservative; however, it is very likely to continue to be useful, probably to quantify pan-Arctic denitrification, possibly also to help quantify dense water formation rates, where that process happens in denitrifying shelf seas. This continuing – albeit different – usefulness of the N : P ratio relies on retention of single salinity and

In terms of

We envisage that sustained measurement of suitable tracers around the Arctic boundary has the potential to further our quantification and understanding of key processes, variability, and timescales and to help mitigate the scarcity of observations in the Arctic Ocean interior. More (and more reliable) tracers are needed, more observations of more traditional tracers are needed through the water column (from surface to sea bed), more of those observations are needed in seasons outside summer and autumn, and we need better understanding of Arctic Ocean biogeochemical processes.

All data used in the analysis presented here are available from the original authors. See Sect.

AF conducted the analysis and prepared the paper. SB, ACNG and STV assisted with the analysis and preparation of the paper. TT and STV assembled the data used, and TT assisted with the analysis.

The authors declare that they have no conflict of interest.

Alberto C. Naveira Garabato acknowledges the support of the Royal Society and the Wolfson Foundation.

This study was funded by the U.K. Natural Environment Research Council as a contribution to the TEA-COSI (The Environment of the Arctic Climate, Ocean and Sea Ice) project grant no. NE/I028947/1.

This paper was edited by Christian Haas and reviewed by Thomas Armitage and Wilken-Jon von Appen.