BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-13-5259-2016 High-resolution neodymium characterization along the Mediterranean margins and modelling of εNd distribution in the Mediterranean basinsAyacheMohamedmohamed.ayache@lsce.ipsl.frhttps://orcid.org/0000-0002-2965-3377DutayJean-Claudehttps://orcid.org/0000-0003-3306-9015ArsouzeThomasRévillonSidoniehttps://orcid.org/0000-0001-8370-4545BeuvierJonathanJeandelCatherineLaboratoire des Sciences du Climat et de l'Environnement LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, FranceENSTA ParisTech, Université Paris-Saclay, 828 bd des Maréchaux, 91762 Palaiseau CEDEX, FranceLaboratoire de Météorologie Dynamique, École Polytechnique, Palaiseau, FranceSEDISOR/UMR6538 “Laboratoire Domaines Océaniques”, IUEM, CNRS-UBO, Plouzané, FranceMercator-Océan, Ramonville Saint-Agne, FranceMétéo-France, Toulouse, FranceLEGOS, Université de Toulouse, CNRS, CNES, IRD, UPS, Toulouse, FranceMohamed Ayache (mohamed.ayache@lsce.ipsl.fr)22September201613185259527624March20165April201612August20168September2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/13/5259/2016/bg-13-5259-2016.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/13/5259/2016/bg-13-5259-2016.pdf
An extensive compilation of published neodymium (Nd) concentrations and
isotopic compositions (Nd IC) was realized in order to establish a new
database and a map (using a high-resolution geological map of the area) of
the distribution of these parameters for all the Mediterranean margins. Data
were extracted from different kinds of samples: river solid discharge
deposited on the shelf, sedimentary material collected on the margin or
geological material outcropping above or close to a margin. Additional
analyses of surface sediments were done in order to improve this data set in
key areas (e.g. Sicilian strait).
The Mediterranean margin Nd isotopic signatures vary from non-radiogenic
values around the Gulf of Lion, (εNd values ∼-11) to
radiogenic values around the Aegean and the Levantine sub-basins up to +6.
Using a high-resolution regional oceanic model (1/12∘ of horizontal-resolution), εNd distribution was simulated for the first
time in the Mediterranean Sea.
The high resolution of the model provides a unique opportunity to represent
a realistic thermohaline circulation in the basin and thus apprehend the
processes governing the Nd isotope distribution in the marine environment.
Results are consistent with the preceding conclusions on boundary exchange
(BE) as an important process in the Nd oceanic cycle. Nevertheless this
approach simulates a too-radiogenic value in the Mediterranean Sea; this bias
will likely be corrected once the dust and river inputs will be included in
the model.
This work highlights that a significant interannual variability of
εNd distribution in seawater could occur. In particular,
important hydrological events such as the Eastern Mediterranean Transient (EMT),
associated with deep water formed in the Aegean sub-basin, could
induce a shift in εNd at deep/intermediate depths that could
be noticeable in the eastern part of the basin. This underlines that the
temporal and geographical variations of εNd could represent
an interesting insight of Nd as tracer of the Mediterranean Sea circulation,
in particular in the context of palaeo-oceanographic applications.
Introduction
The Mediterranean Sea is a semi-enclosed sea of great interest because it is
submitted to large range of dynamical processes and interactions, such as
strong air–sea exchanges leading to open-sea deep-water convection feeding a
thermohaline circulation cell , strait
transports and dynamics or cross-shore exchanges. From a biogeochemical
perspective, it is a region receiving the highest aerosol loads owing to air
masses carrying numerous and various aerosol types
, where oligotrophy occurs and with a
characteristic dynamic of the deep chlorophyll maximum .
Under the stress of the global change and anthropogenic forcing,
understanding the functioning of the Mediterranean Sea and quantifying the
biogeochemical cycles is a priority .
Neodymium (Nd) is a Rare Earth Element (REE) with seven naturally occurring
isotopes. The radiogenic isotope 143Nd is produced by the radioactive
α-decay of 147Sm. At the continent surface, the Nd isotopic
composition (usually expressed as εNd
εNd= [(143Nd/144Nd)sample/(143Nd/144Nd)CHUR- 1] × 104,
where (143Nd/144Nd)CHUR= 0512638 is the averaged
earth value .
of a given material) is a function of the
Sm / Nd ratio characterizing this material, which is primarily a function of
its age and lithology. On a global scale, it is higher in the Earth's mantle
compared to its crust. As a consequence, the εNd of the
continents presents a heterogeneous distribution .
Nd residence time ranges from 700 to 1500 years in the global ocean
e.g., long enough to be transported within the global
thermohaline circulation system and short enough to avoid complete
homogenization. Therefore, εNd is often considered to be a
“quasi-conservative” tracer. In other words, εNd values of
the water masses could be conserved up to long distances from the source of
lithogenic inputs. In such a context, it could be used to tag water masses with
distinct isotopic compositions in order to constrain water mass mixing and
pathways, as well as the thermohaline circulation in modern and palaeo ocean
circulation e.g.. However, because Nd is
particle reactive, Nd parameters are also successfully used to study Nd
exchange between dissolved and particulate phases .
Nd sources to the ocean are lithogenic, and the mean εNd of
an oceanic basin is representative of the surrounding continents
. During the last few years, significant progress has been
made in understanding how different water masses acquire their Nd IC
(Isotopic Composition). In the early 2000s, and
suggested that exchange of Nd between the sediments
deposited on the oceanic margins and the waters flowing along these margins,
called the “boundary exchange” (BE) was the missing Nd source that could
balance both the concentration and isotopic distributions of Nd on regional
and world scales. Since these pioneer works, many studies have confirmed this
hypothesis .
The modelling studies have reached the same
conclusions on the relative importance of the BE on the Nd oceanic cycle on
the global scale, although dust and river inputs could locally affect the
surface waters, such as off the Sahara..
The Nd influx brought by the Atlantic inflow in the Strait of Gibraltar is
smaller than the Nd outflux exiting with the Mediterranean outflow
. The εNd value
of the Mediterranean outflow was estimated to be -9.4 , which
is higher than that of the Atlantic inflow (εNd=-11.8;
). Thus, a source of radiogenic Nd is required to balance these fluxes.
and proposed that the additional Nd
source might be the partial dissolution of river particles and/or aeolian
particles. argued that the missing source might rather be
of marine origin. suggested that the Black Sea was a net
source to the Mediterranean Sea. Based on a two-box model
suggested that the εNd in the Ligurian sub-basin deep waters
required an exchange involving 30 ± 20 % of the sinking particles of
atmospheric origin. Finally, proposed that the missing
term could be sediments deposited on the margins. In other words, the origin
of this radiogenic input remains unclear. The present study aims to compile
data and develop modelling tools for clarifying this issue.
The filled contours indicate the geological province limit based on
geological age (i.e. each colour represents an age) from a high-resolution digital geological map (http://www.geologie.ens.fr/spiplabocnrs/spip.php?rubrique67) while the
circles filled in blue represent the location of the discrete data compiled
from the EarthChem database (see Supplement 1), and in red the location of the
stations correspond to the sediments analysed as part of the present work.
The circulation of the Mediterranean Sea is driven by the fact that the mean
evaporation exceeds the mean precipitation, leading to a density increase
along surface water mass paths and subsequent strong convective events in
winter. The main deep-water sources are located in the Gulf of Lion (south
of France) for the western Mediterranean Sea (WMed), and the Adriatic
sub-basin for the eastern Mediterranean Sea (EMed; ). In the
mid-1990s a shift in the deep-water formation site occurred during the
Eastern Mediterranean Transient (EMT) events. The EMT describes a temporary
change in the Eastern Mediterranean Deep Water (EMDW) formation site that
switched from the Adriatic to the Aegean sub-basin .
The new source has produced large quantities of very dense water masses, in
particular the Cretan Deep Water (CDW) that overflowed through the Cretan Arc
straits and subsequently filled the eastern Mediterranean with waters denser
than the previously existing deep and bottom water. However, EMT was revealed
using hydrographic and anthropogenic tracers such as CFC and 3H.
Both are transients and are not imprinted in the sediments. Establishing the
occurrence of a similar EMT event in the past would require the identification
of proxies that clearly identify the distribution and circulation of the
different water masses, which is memorized in the sediments.
demonstrated that the Nd isotopic signature is more
conservative than the salinity in the Mediterranean Sea, the latter being
strongly affected by the evaporation. In addition, these authors revealed
that the Mediterranean water masses are well distinguished by their Nd
isotopic signatures. The Mediterranean Sea makes an excellent “laboratory
test” basin for studying the potential εNd distribution
variations as it is a semi-enclosed basin with a quite short residence time
of the waters (50–100 years; ). This paper also aims to
investigate how EMT events affect the Nd distribution in the Mediterranean
basin in order to estimate the potential of this tracer to characterize from
palaeo archive (e.g. corals, foraminifera) the occurrence of such event in
the past. Modelling represents an appropriate tool to address this question.
In this study, we developed a new modelling platform for simulating Nd
isotopic composition at high resolution in the Mediterranean basin.
Extrapolated map providing a picture of the Nd signature of all the
margins surrounding the Mediterranean Sea. (a) Low-resolution configuration
ORCA2 (reproduced from ) and (b) high-resolution
configuration NEMO-MED12 (this work). Hatched areas correspond to
uncharacterized areas in the published literature (before 2007) as done by
.
First, the results of a dense compilation of the concentrations and isotopic
compositions of the different materials that constitute the Mediterranean
margins, which are expected to interact with the water masses, are presented.
This high-resolution mapping was established using a detailed geological map,
providing the most realistic representation of the Mediterranean geology
existing so far (see Figs. and , and Supplement 1).
The approach by to simulate the BE was evaluated and generally
accepted by the scientific community (e.g. ), following
this protocol made up for the global scale, we implemented the neodymium in a
high-resolution regional model (NEMO-MED12) developed for the Mediterranean Sea. We
used dissolved εNd data compiled by ,
, to evaluate the ability of this model to
reproduce the main features of the circulation and mixing of the Mediterranean Sea
water masses for which Nd signatures are known. These tools provided
perspectives on (i) the εNd distribution in the whole Mediterranean Sea, (ii) the impact of the interannual variability of the thermohaline
circulation (e.g. EMT event) on the modelled εNd
distribution, and (iii) high resolution of the geological field on the one
hand and the model on the other hand can reveal possible local
heterogeneities which could reflect local BE effects.
This study is part of the work carried out to assess the robustness of the
NEMO-MED12 model, used to study the thermohaline circulation and the
biogeochemical cycles in the Mediterranean Sea, and it improves our ability to
predict the future evolution of this basin under increasing anthropogenic
pressure .
Data compilation and representation on the Mediterranean margins
Here we present the data compilation procedure and resulting map (Fig. )
allowing us to characterize the Nd isotopic signatures and concentrations of
all the margins surrounding the Mediterranean Sea.
Our approach was to prioritize the use of data directly measured in the
outcropping sediments or geological fields, as proposed by
. An extensive compilation of all published Nd
concentrations and isotopic values was made using the EarthChem database
(http://www.earthchem.org) with a zoomed-in image of the Mediterranean region (latitude
between 28 to 48∘ N, and between 10∘ W and
40∘ E in longitude). This yielded assets of more than 14 200 discrete
data reported in Supplement 1 and located in Fig. . When data were
missing in crucial areas (e.g. Strait of Sicily), we directly measured them
on core-top sediments.
Finally, we used a high-resolution numerical geological map to
interpolate/extrapolate data from similar geological areas. Below we briefly
discuss the pros and cons of this latter approach.
Sediment core tops and erodible material data
Surface sediments collected on the shelf or the slope were our first choice
because they provide direct information on the geochemical and isotopic
characteristics of the material in contact with the water masses. Sediments
deposited during the recent Holocene were taken into consideration. When
surface sediment data were missing, we took into account the Nd parameters of
erodible material deposited along the coasts . The EarthChem
database provides a good spatial covering in the northern coastline
(Fig. ), in contrast to the southern coastline (e.g. Algerian coast),
while there are no published data for the Tunisian, Libyan and Egyptian
coasts. We therefore distinguished between the two cases Sects. 2.1.1 and 2.1.2.
Areas with high spatial resolution of available data
For this kind of region (i.e. Italian coast) and for each geological
province, we carefully established the extent to which we could extrapolate
the measured values to the whole province. The average εNd
values for each geological province were calculated, taking into account the
number of cores collected in a given area, and the geochemical
characteristics of the analysed sediments. We neglected data for specific
areas like very small volcanoes which were not representative of the geochemical and
isotopic characteristics of the region, such as in the Strait of Sicily
(see Supplement 1 reports the full treatment, results and
uncertainties). This approach was to generate relatively robust Nd isotopic
signatures characterizing the whole of the northern Mediterranean Sea margins, with
a standard deviation less than 2εNd units in most cases.
Areas with low spatial resolution of available data
For the southern coast, we gathered sediment samples from miscellaneous
origins. Those were collected during the cruises of ETNA80, DEDALE and NOE
near the Tunisian, Libyan and the Egyptian coasts respectively (see Table 1).
In the Strait of Sicily samples were collected along two transects,
Sciacca–Pantelleria (SP) and Pozzallo–Malta (PM),
perpendicular to the southern coast of Sicily . The
sampling site identifications, depths, collection dates and positions are
compiled in Table 1.
Sediment samples were analysed for Nd radiogenic isotopes. About 100 mg of
samples were weighted and dissolved in teflon beakers in a mixture of
ultrapure quartex HF (24N), HNO3 (14N) and HClO4 (12N) for 4 days at
160 ∘C on a hot plate. After evaporation to dryness samples were
dissolved in aqua regia and heated for 24 h at 130 ∘C. Nd
fractions were chemically separated following conventional column chemistry
procedures described in . Nd isotope compositions were
measured in static mode on a Thermo TRITON at the PSO (Pole de
Spectrométrie Océan) in Brest, France. All measured Nd ratios were
normalized to 146Nd / 144Nd = 0.7219. During the course of analysis,
Nd standard solution La Jolla gave 0.511854 ± 0.000008 (2α; n= 28,
recommended value 0.511850) and JNdi gave 0.512099 ± 0.000010 (2α;
n= 6, recommended value 0.512100). Procedural blanks were all below 200 pg and
therefore negligible in all cases.
River and dust inputs
Dissolved Nd in river water is efficiently removed from solution by
coagulation of colloids during the estuarine mixing. The recent compilation
of confirmed that on a global scale 71.8 ± 16 % of
dissolved riverine Nd is removed by this process, in agreement with preceding
works e.g.. In addition, these authors evidenced
that lithogenic Nd is released at higher salinities by the suspended
particulate material discharged by the river. Globally, this could represent
5700 ± 2600 Mg of dissolved Nd annually brought to the ocean by this
mechanism, a flux 6–17 times larger than the dissolved one, and 8 to 21 times
larger than the atmospheric flux, assuming 2 % dust dissolution. Note that
the mechanism evidenced in the Amazon estuary by does not
describe all the processes likely to affect the sediments deposited on the
shelves and margins. The hypothesis was modelled by and
. Dust input is also difficult to constrain: while flux is
sporadic and hard to characterize, establishing the fraction dissolved at the
air–sea interface is also challenging.
Coordinates of the studied cores together with water depth.
CruiseYearsLongitudeLatitudeDepthεNd(∘ E)(∘ N)(m)ETNA80198011.4836.30263-10.0913.4433,23736-10.92DEDALE198725.5933.513020-8.15NOE198430.0132.191465-4.4930.131.53495-3.92Strait of Sicily200312.5737.3029.5-11.6714.3736.3687.4-11.0514.2236.16488.2-11.2212.3236.56117-8.06
The main river systems of the Mediterranean basin are the Nile, Po and Rhone.
River plume extensions were established using maps and satellite images from
data banks provided by .
Nd input derived from the Nile river water and/or particles could be
transported eastward and northward to the Rhodes Gyre where the LIW is
formed. suggest that the most significant radiogenic Nd
source to the EMed is partially dissolved Nile River particles, radiogenic Nd
supplies to the eastern basin being formed by dissolved and particulate loads
(εNd of about ∼-4).
have studied the potential impact of river inputs on the Nd
isotopic composition of the WMed. The Rhone transports 80 % of the solid
riverine discharge into the north-western Mediterranean Sea
. According to the Rhone dissolved water
and superficial sediments display an average εNd value of
-10.2 ± 0.5.
In contrast, the Po river is less documented. The geographical extension of
Po river drainage basin was defined using the digital geological map
referenced above, and database. These information allowed
us to extract the Nd isotopic signatures from the work of
, and . These studies
are the most representative of the Po drainage basin so far.
Finally, we included aeolian inputs in our compilation. To this extent, the
review of was of great help. Indeed, this work presents
a review of bulk composition data of northern dust inputs, their potential
sediment sources and their elemental, isotope and mineralogical
characteristics. Actually, these aeolian data will not be immediately used in
our modelling approach. However, we considered relevant to present them with
the remaining data, which allowed us to propose the most comprehensive data set.
All the discrete data extracted from the literature are reported in Supplement 1
and Fig. .
Extrapolation and extrapolation
Extrapolating the collected discrete data is required to allocate the margins
with continuous Nd concentration and isotopic compositions. In other words,
we attribute an isotopic signature and Nd concentration to any margin area
liable to be in contact with the waters flowing through. To this end, the
tools developed by for the world margins were adapted for
the Mediterranean Sea.
Because the Nd isotopic composition of any field is closely related to its
geological nature and age we used a high-resolution digital geological map which provides the contours of the fields
(Fig. ) of a given geological age and type
(http://www.geologie.ens.fr/spiplabocnrs/spip.php?rubrique67). This allowed
us to estimate the size of the coastal segments that could provide material
with the same Nd characteristics.
In poorly documented areas, we first considered the age and geochemical
nature of the field . Then we applied the same isotopic
signature as similar fields of the same age based on the Nd model-age
relationships .
This was thoroughly done by checking the geology, geochemistry and Nd
signatures of the fields identified as the source of deposited material. Note
that the resolution of the available data in the Mediterranean basin is
relatively high, reducing the uncertainties of the approach described above
(see Sect. 2, Supplement 1), Fig. b reveals the improvement allowed
by this approach by comparing the high-resolution patchwork of field
documented to an extraction of the Mediterranean Sea basin
(Fig. a) from the global low-resolution distribution of (see Sect. 4.1).
Modelling the Nd isotopic compositionDescription of the model
We use the free-surface ocean circulation model NEMO (Nucleus for European
Modelling of the Ocean) in a regional configuration called
NEMO-MED12 , already used for biogeochemical studies
. This model uses
the standard ORCA grid of NEMO at 1/12∘ resolution. This corresponds
to a grid cell size in the Mediterranean area varying with latitude between
6 and 8 km, from 46 to 30∘ N and it extends into the
Atlantic Ocean to 11∘ W (buffer zone). Vertical resolution varies
with depth from ΔZ= 1 m at the surface to ΔZ= 450 m at the
bottom with 35 levels in the first 1000 m (50 levels in total).
Daily mean fields of momentum, freshwater flux (evaporation minus
precipitation) and net heat flux from the high-resolution atmospheric data
set (ARPERA) are used for the air–sea fluxes
. The heat flux is applied with a
retroaction term using the ERA-40 sea surface temperature (SST).
The initial state (temperature, salinity) for the Mediterranean Sea came from
the MedAtlas-II climatology weighted by a
low-pass filter with a time window of 10 years using the MedAtlas data
covering the 1955–1965 period, following . For the
temperature and salinity in the buffer zone (west of the Strait of Gibraltar), the
initial state is prescribed from the 2005 World Ocean Atlas
. River run-off is prescribed from the
interannual data set of . The Black Sea is not explicitly
included in the models, but is rather treated as one of the major freshwater
sources of the Mediterranean Sea located at the Dardanelles strait, with a
flux corresponding to the Dardanelles net budget estimates of .
NEMO-MED12 model simulates the main features of the thermohaline circulation
and mixing of the Mediterranean Sea water masses and their interannual
variability. In particular, the propagation of the Levantine Intermediate
Water (LIW) from the eastern to the western basin is produced with realistic
timescale compared to the observations . However, some aspects
of the model still need to be improved. For example, the formation of Adriatic
Deep Water (AdDW) is too weak, leading to a too-low contribution to the EMDW
in the Ionian sub-basin . The
atmospheric forcing used by includes some modifications
to improve dense water fluxes through the Cretan Arc during the EMT. As
established in the previous version of NEMO-MED8 (1/8∘ of horizontal
resolution, ), the model is able to reproduce a transient
deep-water formation as observed for the EMT, but the simulated transient
produced less Eastern Mediterranean Deep Water (EMDW).
later performed a sensitivity test with modified forcing. The ARPERA forcings
were modified over the Aegean sub-basin, by increasing daily water loss by
1.5 mm, daily surface heat loss by 40 W m-2, and the daily wind stress
modulus by 0.02 N m-2 during November to March in the winters of
1991–1992 and 1992–1993, as done by to study deep convection
in the Gulf of Lion. This resulted in average wintertime increases in heat
loss (+18 %), water loss (+41 %) and wind intensity (+17 %) over the
Aegean sub-basin. These changes generate an improved circulation that
satisfyingly reproduced the formation and renewal of the deep water in the
eastern basin during the EMT event . These performances of
the dynamical model have to be kept in mind when analysing the Nd simulations.
The tracer model
Using different modelling approaches, and
have shown that the exchange between the continental
margins and seawater, the boundary exchange (BE) represents the major
source of Nd on the global scale. This source represents more than 90 % of
the total input, whereas dissolved riverine and dust inputs could only be
significant in the upper 500 m. However, so far only little is known
about the importance of the BE in semi-enclosed and/or interiors basins
like the Mediterranean Sea, where atmospheric and river fluxes could also have
significant impacts on the εNd distribution.
As a first approach, we chose to simulate only the Nd isotopic composition
(εNd) in order to test the BE hypothesis in the
Mediterranean Sea . This approach does not require
explicitly simulating the Nd concentration, allowing us to focus on the
timescale of the process studied. As in ,
εNd is implemented in the model as a passive conservative
tracer which does not affect ocean circulation. It is transported into the
Mediterranean Sea by NEMO-MED12 physical fields using a classical
advection–diffusion equation, including the sources and sinks (SMS term,
eq1). The rate of change of oceanic Nd isotopic composition is as follows:
δεNdδt=SεNd-U⋅∇εNd+∇⋅K∇εNd,
where S(εNd) represents the SMS term, U⋅∇εNd
is the three-dimensional advection and ∇⋅ (K∇εNd)
is the lateral and vertical diffusion of εNd.
Since εNd is a passive tracer, simulations could be run in
offline mode using the pre-computed transport fields (U, V, W) from the
NEMO-MED12 dynamical model . Physical forcing fields are
read daily and interpolated at each time step of 20 min. Offline simulations are
performed for computational efficiency, allowing many sensitivity tests on the
SMS term parameterization. The same approach was used by
to simulate the mantle and crustal helium isotope signature, by
to model the anthropogenic tritium invasion, and by
to simulate CFCs and anthropogenic carbon storage.
Map of the εNdmargin used in the model
simulation, made by interpolation of Fig. b on the oceanic margins
of the Mediterranean Sea (see Sect. 2).
The only SMS term taken into account in the present study is BE
. It is parameterized by a relaxing equation
between the ocean and the continental margin:
SεNd=1/τεNdmargin-εNd⋅maskmargin,
where τ is the characteristic relaxing time (i.e. the characteristic
time needed to transfer isotopic properties from the continental margin to
the ocean), εNd is the Nd isotopic composition of seawater,
εNdmargin is the value of the material deposited along the
continental margin (see Sect. 2), and maskmargin is the percentage
of continental margin in the grid box which represents the proportion of the
surface in the grid where the BE process occurs. This quantity is estimated
from the high-resolution bathymetry of the 10th version of the
Mercator-LEGOS bathymetry at a resolution of 30′′× 30′′.
The topographic extension of the oceanic margins of the Mediterranean Sea has
been chosen to the ∼ 540 m (Fig. ) following the margin
definition used to model the iron cycle in the Mediterranean Sea by .
The exchanges of the Nd with the Atlantic Ocean are specified through a
buffer zone between 11∘ W and the Strait of Gibraltar.
εNd values in the buffer zone are prescribed from
observation using NE Atl. MED-15 vertical profile from .
We established some sensitivity experiments regarding the optimal value of τ
in the Mediterranean basin . In this aim, six
tests were performed, referred to as EXP1, EXP2, EXP3 and EXP4 with τ= 1,
3 months, 6 months and 1 year respectively. As surface ocean
currents are generally more dynamic than deep ones, providing more energy for
sediment–seawater interactions, we realized an additional simulation (EXP5)
wherein τ increases exponentially with depth from 1 month at the
surface to 1 year at 600 m depth. We also explored the possibility that our
BE parameterization might be dependent on the mineralogical maturity of
margin sediments (e.g. granitic vs. basaltic). Hence, relaxing time τ
in EXP6 is varying linearly on a timescale of 1 month for the most radiogenic
isotopic signature (i.e. εNd=+6 on the extreme east of
the Mediterranean margin) to 1 years for the most non-radiogenic values
(i.e. εNd=-12 along the Spanish coast).
Model–data comparison for the six simulations performed with different
relaxing time at the steady state (see Table 3) and the in situ data from
, and :
(a) model–data correlation, red line is the linear regression from
EXP2. Diagonal dashed lines are lines εNd (modelled) =εNd (data),
εNd (modelled) =εNd (data) + 3εNd and
εNd (modelled) =εNd (data) - 3. (b) Model–data comparison
as a function of depth, dashed solid line represents the data from
, and .
The simulations were initialized with uniform isotopic composition of
εNd=-7 and integrated to steady state; i.e. the global
averaged drift was less than 10-3εNd per thousand
years, for more than 75 years of spin-up run.
ResultsMap of the outcropping Nd values
Results of the Nd parameter mapping are represented in Fig. , cold
colours represent the old non-radiogenic rocks whereas the warm colours
correspond to the recent radiogenic ones.
Tectonic and associated volcanic activities led to the very complex
morphology and geology in the Mediterranean region, comprising small islands
(e.g. Corsica, Cyprus), sub-basins (e.g. Adriatic, Aegean and Tyrrhenian),
and many narrow straits (e.g. Strait of Sicily, Otranto Passage). This
particular context prevents the use of low-resolution grid to represent
this region properly. This motivated the realization of the high-resolution
(1/12∘× 1/12∘) version of the Nd isotopic signature
(Fig. b) and Nd concentration (see Supplement 5) for this basin.
The general trend is that the margin Nd isotopic signatures vary from
non-radiogenic values in the WMed, to radiogenic values when reaching the
Aegean and Egyptians coasts, the most radiogenic fields (εNd
up to +6) being located around the eastern border of the Levantine sub-basins,
and in the volcanic region of the south of Italy (Fig. b).
In contrast, the southern Sicilian fields and the northern Alboran sub-basin are
characterized by the most negative isotopic signature (values around -12).
The Algerian, Tunisian, French and Spanish coasts display relatively
homogeneous values between -11.5 and -10. Such east–west gradient of Nd
isotopic signature is also observed in the seawater data, where poorly
radiogenic waters from the Atlantic are progressively shifted toward more
radiogenic values in the Levantine basin .
Output of model from EXP2 (t= 3 months) at the steady state. Upper
panel: horizontal maps for surface waters (a), intermediate waters (b),
and deep waters (c). Lower panel E–W section in WMed (d),
and EMed (e), whereas colour-filled dots represent in situ observations
from , and . Both use
the same colour scale.
All the details revealed by this new high-resolution map will be used to set
boundary conditions in the regional simulation (see Sect. 3, Fig. ).
The characteristic margin-to-ocean exchange time
We first explored the impact of changing the value of the relaxing time on
the εNd distribution in the Mediterranean Sea. This was made following
the strategy adopted by in the North Atlantic basin,
although NEMO-MED12 model has higher horizontal and vertical resolutions
(1/12∘ in this study compared to 1/4∘ in ).
The results of these different sensitivity tests are compared with in situ
observations collected by , and
using correlation plots, coloured maps and sections (Figs. and ).
The simulated εNd distributions in EXP2 and EXP3 (relaxing
time of 3 and 6 months respectively) present a better correlation with
in situ data relative to the other experiments, with correlation coefficients
close to 0.75 and 0.60 respectively (Table 3). The difference between in situ
data (dashed line) and the different sensitivity experiments as a function of
depth (Fig. b) reveals that EXP2 provides the best agreement with
observations, despite a slight overestimation of εNd between
0.3 and 2εNd units. The data–model differences are more
important for the other experiments which produced too-radiogenic simulations
(of more than 2εNd units). The horizontal distribution of
εNd (Supplement 2–4) confirms this statistical
correlation, showing that only EXP2 and EXP3 produced reasonable east–west
gradients of εNd. EXP1 generated too-pronounced
εNd geographical gradients, particularly in surface waters
along the continental margins, suggesting an overestimation of the exchange
compared to the transport. On the opposite a simulation with a relaxing time
of 1 year (EXP4) leads to a homogeneous εNd distribution in
surface and deep waters with a low data-model correlation, indicating an
underestimation of the boundary exchange process. EXP5 (τ increases
with depth) conducted to a strong gradient over the entire water column in
WMed, showing that surface-to-deep variation of the BE rate was likely
overestimated in this simulation. EXP6 displayed a realistic E–W gradient in
the surface waters, but a too-homogeneous εNd signal in the
intermediate and deep waters, suggesting that the BE rate seems weakly
affected by the lithology of the margin sediments. Finally, we consider that
the characteristic exchange time providing the best agreement with
observations is close to 3 months. This value is consistent with the results
obtained by with its simulation of the North Atlantic
area. Therefore we will only consider EXP2 for the rest of our analysis.
The εNd distribution
The monthly-averaged εNd horizontal distributions resulting
from EXP2 for the surface waters (0–200 m), the intermediate waters (200–600 m)
and the deep waters (600–3500 m), are represented in Fig. a–c respectively, together with the data
from , and . The model
results are extracted after the steady state (in 1987) of the simulation and
considered representative of a pre-EMT situation (i.e. EMT, ).
The model correctly simulates the pronounced εNd E–W
gradient characterizing the surface waters (Fig. a). The values
simulated in the WMed and eastern Levantine basin are consistent with the
observations while the simulated values in the Aegean and central Levantine
basin tend to be too radiogenic. At intermediate depths, both modelled and
observed E–W gradients are less pronounced than at the surface (Fig. b).
However averaged simulated values are relatively too radiogenic
at the intermediate level (-5.8 compared to -9.4 ± 0.69, Table 2). Especially
high εNd signatures are simulated in the Aegean sub-basin,
over the Strait of Sicily and in the Tyrrhenian sub-basins (Fig. b).
A significant model–data disagreement was found in the Alboran sub-basin
which largely overestimates the observations. The εNd distribution
in the deep waters is relatively homogeneous over the whole basin except in
the Aegean sub-basin and Strait of Sicily (Fig. c).
Mean εNd for the Mediterranean Sea, from EXP3 and for the
in situ data from .
Model In situ data IntermediateAverageIntermediateAveragewatersall depthswatersall depthsMediterranean Sea-5.8-6.2-7.6 ± 1.37-7.8 ± 1.54Eastern basin (EMed)-4.7-5.1-7 ± 0.85-7.1 ± 1.08Western basin (WMed)-5.8-6.3-9.4 ± 0.69-9.6 ± 0.48
Comparison of average vertical profiles of Nd isotopic signature (Nd-IC)
from EXP2 (t= 3 months) in the whole Mediterranean Sea. Model results
are in blue, while red indicates the in situ data from ,
and .
The Levantine Intermediate Water mass (LIW) is well identified by its marked
radiogenic signature. LIW is produced in the Levantine sub-basin before
passing Crete at 28∘ E, where measured εNd values
reach -5 (Fig. ). The εNd isotopic signature is
well identified over the entire LIW trajectory at the intermediate level
(between 200 and 600 m depth), with values around -4.8 in the Algerian
sub-basin and up to -5.7 in the Alboran sub-basin (Fig. d). The
resolution of the available data hardly allows us to evaluate the model
performance for this water mass; nevertheless, station 74 (33∘7′ N,
33∘5′ E) in the eastern Levantine basin exhibits a discernible
radiogenic signal associated to LIW (more pronounced than the modelled one),
while station 51 (33.5∘ N, 27∘ E) in the western Levantine
basin exposes a relatively homogeneous vertical isotopic signature. The
surface waters originating from the Atlantic Ocean (Atlantic Waters, AW) are
characterized by the most negative signature (value around -9) and are
transported over all the Mediterranean Sea, allowing them to be clearly identified. The
εNd signatures of the deep-water mass display values
around -6.5, consistent with the observations available in the eastern basin (Fig. e).
Except in the Alboran sub-basin, where pronounced mismatches are simulated
between the model and the observations, the model captures the general features of the
vertical profiles of Nd isotopic signatures, especially in the Levantine
sub-basin (averaged over the entire water column), producing a realistic and
significant radiogenic signature associated to LIW at the intermediate level
(Fig. ), although the εNd values can be
overestimated in some places by almost 2εNd units.
Summary of the main characteristics for each experience.
ExperienceRelaxing time τRegressioncoefficientfor data/modelpointsEXP11 month0.32EXP23 months0.75EXP36 months0.60EXP41 year0.34EXP5τ varying vertically from 1 month at the surface to 10 months at 540 m0.27EXP6τ 1 month (max εNdmargin) to 1 year (min εNdmargin)0.39
The εNd evolution from EXP2 (τ= 3 months) at the
intermediate level in (a) (average depth between 200 and 600 m), and
for the deep levels in (b) (average depth between 600 and 3500 m).
In the Gulf of Lion (yellow), Algerian sub-basin (blue), Levantine s-b
(cyan), Ionian s-b (green), Strait of Sicily (red), Tyrrhenian s-b (black).
The interannual variability
In this section, we analyse inter annual variations on the redistribution of
εNd over the Mediterranean basin, with a special focus on
the possible impact of the EMT events. The evolution of the monthly averaged
εNd at the intermediate level (between 200 and 600 m) in
different “boxes” following the LIW trajectory from the Levantine sub-basin
to the Algerian sub-basin (including Ionian, Strait of Sicily, Tyrrhenian, and
Gulf of Lion) is represented in Fig. for the 40 years of the
simulation. It shows that εNd signatures vary seasonally
with maximum amplitude of 0.2εNd units. The EMT event
significantly impacts the εNd signature at the global scale
of the Mediterranean Sea. After 1992, which is referred to as the beginning of the EMT
event, an important change of εNd distributions is simulated
over all the Mediterranean Sea, with regional values shifted by almost
0.5εNd units.
The drastic change caused by the EMT event at the beginning of the 1990s is further
illustrated by showing the differences of εNd
distributions between the pre-EMT situations in 1987 and the subsequent years
up to 2010. The analysis on different horizontal levels (Fig. ), as
well as along the E–W sections (Fig. ), provides a better understanding
of the source of ventilation for the interior of the Mediterranean Sea, and
the connection between the surface, intermediate and deep-water redistribution.
Horizontal maps showing the difference relative to 1987 from EXP2
(τ= 3 month), in the left column for the surface level (0–200 m), in the
middle column for the intermediate layer (between 250–600 m, and for the
deep layer in the right column (600–3500 m).
Colour-filled contours represent simulated Nd isotopic composition
for the WMed in the left column, and the EMed sections are shown in the right
column. The first line show the situation in 1987 (pre-EMT), the others
sections show the difference in the Nd-IC between 1995 and 2010 (post-EMT
period) corrected to the 1987 situation.
In comparison with the steady-state situation for the Mediterranean Sea
circulation (pre-EMT), the surface waters are relatively less radiogenic in
the Levantine sub-basin, the Algerian, and the Alboran sub-basins between 1995
and 1999 (Fig. a and b). After 2001 these surface
waters became more radiogenic over the whole basin. At the intermediate level
only the EMed presents a less radiogenic signature in 1995; indeed the
εNd are more radiogenic over the entire basin after 2001
(Fig. f–h). The deep waters are
globally more radiogenic between 1987 and 2010, especially in the EMed where
increase of 1 units of εNd are simulated around the Aegean
sub-basin. The vertical section illustrates the important penetration of the
surface and intermediate waters characterized with radiogenic
εNd into the deep waters near the Cretan Arc as a
consequence of the EMT that shifted the Nd isotopic signature by almost
+1.3εNd units in the bottom waters (Fig. b). This
radiogenic signal is maximum in 1995 at the bottom water around the Cretan
Arc near 26∘E, and for the following years (i.e. 1997, 1999, 2005 and 2010)
propagates in the deep waters of the whole Levantine sub-basin, which
typically becomes more radiogenic of +0.5 of εNd (Fig. ).
The amplifying tracer penetration caused by the EMT event generates less
radiogenic values at the LIW layer in the EMed in 1995 and 1999, because this
water is mixed with upwelled pre-EMT less radiogenic water masses.
In contrast the simulated values become globally more radiogenic in the
WMed. The radiogenic transient signal enters inside the western basin through
the LIW outflow (up to +0.6εNd unit) and gradually
penetrates into the deep water through time. However the most
εNd shift was simulates in the Levantine sub-basin deep
water with more than 1.3 unit change (Figs. b and ).
Discussion
The high-resolution simulation presented here provides a too-radiogenic
signature of Nd isotopic signature in the Mediterranean Sea; nevertheless this approach
confirms the primordial role of the BE as the major source of Nd in the
marine environment, similar to what has been previously demonstrated for the
global ocean and the Atlantic basin .
This reinforces the preceding conclusions of BE as a major process in the Nd
oceanic cycle, even at regional scale and in a semi-enclosed basin such as the
Mediterranean basin. Although the processes leading to BE are still not fully
understood , the resulting timescale is of the order
of few months, in agreement with . This timescale is also
consistent with the kinetic rates of Nd release from basaltic material during
the batch experiments conducted by . It is also consistent
with the field data and their Lagrangian modelling developed by
in the highly dynamic south-western Pacific. Taking into
account the lithology of the margin sediments did not improve our
simulations. This requires more laboratory experiments, targeted on the issue
of the nature of the sediments. Nevertheless the comparison with the
available data in the Mediterranean Sea reveals that this approach simulates a slightly
too-radiogenic value in the surface and intermediate waters, especially in
the Aegean and the Alboran sub-basins. The uniquely available observation in
the Alboran sub-basin is located close to the Strait of Gibraltar, and shows
εNd values characteristic of the outflow from the Atlantic
sector. The model fails to reproduce this signal associated to the advection
of water mass of Atlantic origin (see Fig. a), due to a simulated
net water flux input from the Atlantic that stands in the lower range
compared to observations . However, the global radiogenic
bias will likely be corrected once the dust and river inputs are
simulated. Indeed, those could locally affect the surface waters with less
radiogenic values. The main river systems of the Mediterranean basin
(i.e. the Nile, Po and Rhone) are characterized by a wide range of Nd isotopic
signature, with an average εNd value of -10.2 for the Rhone,
and rather radiogenic Nd isotopic ratios for the Nile (εNd∼-4).
The input of Saharan dust has important effects on the
Mediterranean region , where the Nd isotopic compositions
of aerosols range from -9.2 in the eastern part of northern Africa
(e.g. Egypt) to -16 in the central and western parts of northern Africa
. Previous studies suggest that the
εNd distribution at the near surface, for the most part,
reflects river and aerosol inputs . Hence, it is clear
that taking into account dust and river input in future work could
improve the simulation of Nd isotopic distribution in the Mediterranean Sea.
The LIW layer is particularly characterized by the most radiogenic signature
in the intermediate level between 200 and 600 m, which is in good agreement
with in situ observations from especially with the
highest εNd value of -4.8 found at about 200 m in the
easternmost Levantine basin. The LIW represents the principal movement of
water mass from the EMed into the WMed. This LIW signature is conserved in
the WMed, allowing us to study the impact of interannual variability,
including the exceptional events observed in the ventilation of the deep
waters (e.g. EMT) in the whole basin. The too-radiogenic isotopic signature
simulated in the LIW layer at 25∘ E can be explained by the fact that
the LIW are formed NW of Levantine sub-basin near the Cretan Arc, where
the margin IC are about -4, leading to a relatively radiogenic signature as
we consider only the margin Nd source. Also, tritium/helium
and CFC simulations have shown that
the model overestimates the mixing near the Cretan Arc and, as a consequence,
the Levantine sub-basin isotopic signature is overrepresented in this water mass.
The sequence of the EMT events occurring in the EMed at the beginning of the
1990s has completely changed the deep-water mass structure. Different
hypotheses concerning the preconditioning of the EMT and its timing have been
proposed in the literature
.
In our simulation, the εNd distribution between 1995 and 2001 also revealed that
LIW and deep-water signatures were very different from the pre-EMT picture
(see Fig. ). This amplification of mixing caused by the EMT
generates accumulation of radiogenic water at the bottom. The 1995 section
emphasizes the severe impact of the EMT on water mass distribution, which
transfers massive volumes of surface/intermediate waters into the deep
layers, with the highest contributions toward the bottom and south of Crete
(Fig. ), causing a temporary change in the EMDW origin, from the
Adriatic to Aegean sub-basin in 1992–1993. The renewal of the deep-water
masses is very similar to the tritium/helium-3 redistribution observed by
, which is satisfyingly simulated by our regional model
. This gives some more reliability to the evolution of
εNd distribution simulated after the EMT event (Fig. ).
The EMT modifies the characteristics of EMDW in the Levantine sub-basin by
increasing the εNd signature over the entire eastern basin
(Fig. ). Hence the LIW layer is also affected by this
εNd shift, which is next transferred rapidly in the WMed by
the overflow of the Strait of Sicily. The LIW signal then propagates at depth
in the western basin, illustrating how the EMT event modifies water mass
characteristics and potentially affects the formation of deep and bottom water
masses in this sub-basin.
Our results suggest that the shift is more important in the Levantine deep
water, compared to intermediate water where the EMT impact is lower. This
sensitivity test gives a useful diagnostic on the long-term variability of
Mediterranean Sea circulation and demonstrates the potential of Nd to detect a
EMT-like event. However the weak formation of AdDW could affect the simulated
sift of seawater Nd IC in the Ionian deep water.
Conclusions
This study proposes a new map compiled from in situ data with a sufficient
resolution to cover the very complex morphology and geology of the
Mediterranean Sea. This map shows Nd isotopic signatures for all the
Mediterranean Sea margins. The quality of this interpolated map means it can be used
as a continuous source of εNd to make a link between an
ocean circulation model and the tracer inputs from the margins in order to better
understand the thermohaline circulation in modern and palaeo ocean
circulation. This compilation provides a complete picture of the
εNd of the whole Mediterranean margins which could interest
other earth science fields (e.g. solid earth, weathering, tectonic, etc.).
The εNd distribution was simulated using a high-resolution
regional model at 1/12∘ of horizontal resolution (6–8 km). The
boundary exchange (BE) parameterization was performed via a relaxing term
toward the isotopic composition of the margin from this new geological map.
The characteristic margin-to-ocean exchange time is about 3 months in the
Mediterranean Sea, in good agreement with the previous estimation of
in the northern Atlantic basin. The high resolution of the
geological field on the one hand and the model on the other hand has revealed
local heterogeneities attributed to local BE effects that would not be
detected using a coarse-resolution model. This is not confirmed by the data yet but
they could be improved when all the measurements done in the framework of
MedBlack Geotraces cruise are available.
Our next step is therefore to use a fully prognostic coupled
dynamical/biogeochemical model with an explicit representation of all Nd
sources (i.e. atmospheric dusts, dissolved river fluxes, and margin sediment
re-dissolution) and sinks (i.e. scavenging) to simulate the Nd oceanic cycle
in another dedicated study. More in situ data should help to improve
knowledge of Nd and its isotope cycles in the Mediterranean Sea, which will better constrain
the fluxes of solid material and exchange between the continental margin and
open ocean.
The boundary sources with εNd implemented as a passive
conservative has represented an interesting opportunity to explore the
interannual variability on the εNd distribution. Indeed, the
Eastern Mediterranean Transient (EMT) signal from the Aegean sub-basin was
simulated, conducting a significant and measurable evolution of
εNd signal over the whole Mediterranean basin. It confirms
that εNd represents an appropriate proxy to improve our
knowledge on the long-term trend in the Mediterranean Sea circulation, especially to
explore whether EMT-type events occurred in the past .
New Nd palaeo data e.g. or recent
Nd observations collected on corals or foraminifera in the context of the
PaleoMeX (Palaeo Mediterranean Experiment) programme should give an opportunity
to address this question.
Data availability
The data used in this study was obtained from an extensive compilation of all
published Nd concentrations and isotopic values using the EarthChem database
(accessible from http://www.earthchem.org). All supplemental figures and
data references (Sect. 2) can be found in the Supplement.
The Supplement related to this article is available online at doi:10.5194/bg-13-5259-2016-supplement.
Acknowledgements
We would like to thank Koji Suzuki and an anonymous reviewer for their careful
reading of the manuscript and helpful remarks. The authors wish to
acknowledge F. Bassinot, G. Tranchida and P. Censi for sediment samples. We
thank S. Conticelli who kindly answered our questions on geological
features of the Italian coast, which greatly helped for the interpolation.
Hugo Pradalier is acknowledged for his contribution at the beginning of this work.
Edited by: K. Suzuki
Reviewed by: two anonymous referees
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