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Temporal Pattern and Profile of a Coastal‐Deep Sea
Conveyor at a Marginal Deep Oligotrophic Sea
Ronen Alkalay
1,2
, Yishai Weinstein
1
, Barak Herut
2,3
, Tal Ozer
2
, Olga Zlatkin
1
, Tslil Bar
3
,
Ilana Berman‐Frank
3
, and Timor Katz
2
1
Department of Geography and Environment, Bar‐Ilan University, Ramat‐Gan, Israel,
2
Israel Oceanographic &
Limnological Research, Haifa, Israel,
3
Department of Marine Biology, Leon H. Charney School of Marine Sciences,
University of Haifa, Haifa, Israel
Abstract Sediment trap data set and
234
Th profiles (deep water excesses and deficits) reveal that particulate
organic carbon (POC) export at the highly oligotrophic Levantine Sea is dominated by lateral transport from the
nearby margin. These intermediate nepheloid layers (INL) operate at multi‐depth, with the silt‐to‐clay size
particulate matter (PM) fraction transported at water depths of about 100–500 m, while finer fraction arrives also
at deeper depths. The shallow NIL is triggered by winter storms, manipulated by coastal flash floods and shelf
resuspension and assisted by cross‐shore currents, which allow the arrival of PM at a distance of 50 km within
about 10 days. The deeper INL could be related to sediments initially driven to depth by density currents. Our
data show that inter‐annual differences in sediment trap fluxes were related to changes in both the intensity of
coastal floods and current velocity. The frequent observation of deep‐water
234
Th excesses during a (relatively)
low export winter (2018) is related to lessened cleansing of the water column, that is, reduced removal of fine‐
grained PM by sinking coarser‐grained material. These observations highlight the importance of winter storm
intensity in the POC budget of marginal seas like the Levantine Basin (LB) even in areas with limited river
discharge. This further suggests that the anticipated increase in extreme weather events due to the on‐going
climate change should have an impact on this coastal‐deep sea conveyor and on POC export in the LB.
Plain Language Summary We present sediment traps and radioisotope results from the DeepLev
marine station, the first deep water mooring to be deployed in the highly fragile ecosystem of the Levantine
Basin (eastern Mediterranean Sea). Unlike the open ocean, particulate organic carbon export from surface to
deep water is controlled by the transport of particulate matter from the nearby coast/continental shelf. We show
that this land‐sea conveyor is nurtured by flash floods and shelf sediment resuspension, and is further
manipulated by cross‐shore currents. The conveyor operates at multiple depths, with silt and clay transported
sub‐horizontally from the shelf, arriving at DeepLev within ca. 10 days at 100–500 m depth (shallow
intermediate nepheloid layer, INL), while colloids being carried through the whole water column (deep INL).
The latter is probably related to particle‐laden density currents, which flow down submarine canyons, cutting
into the nearby continental slope. Inter‐annual changes in winter events and cross‐shore current velocity result in
significant changes of POC export intensity. This further implies that the predicted increase in the occurrence of
extreme meteorological events could result in an enhanced transport of particular carbon, with important
implications to the POC export in this and other marginal basins.
1. Introduction
Margins are recognized as a significant source of organic carbon to the deep ocean, contributing over 50% of the
particulate organic carbon (POC) in deep ocean sediments, with predominance of terrestrial POC (Berner, 1992;
Buesseler et al., 2010; Danovaro et al., 1999; Hedges & Keil, 1995; Lamborg et al., 2008; Shen et al., 2020;
Wheatcroft et al., 2010). While there is clear evidence for margins' contribution to offshore POC in areas with
large river and drainage area systems (e.g., D. L. Cai et al., 1988; Li et al., 2017; Ludwig et al., 1996; Shen
et al., 2020), less is known about offshore transport of POC in dryer areas, characterized by low discharge.
Notably, it has been argued by different authors that small river systems may contribute a large fraction of the
terrestrial particulate matter (PM) input (e.g., Coynel et al., 2005; Gomez et al., 2003; Wheatcroft et al., 2010);
however, there is a need for further characterization and quantitative comprehension of this transport and its
impact on the marine carbon cycle. Furthermore, the role played by small river systems and their contribution to
RESEARCH ARTICLE
10.1029/2023JC020441
Key Points:
•POC export in the Levantine Basin is
controlled by lateral transport of multi‐
depths intermediate nepheloid
layers (INL)
•The shallow INL carries silt and clay
from the coast and shelf, while the deep
INL hauls colloids related to transport
through submarine canyons
•Inter‐annual variability in winter event
intensity results in lateral transport and
vertical export variability
Correspondence to:
R. Alkalay,
ronen.alkalay@gmail.com
Citation:
Alkalay, R., Weinstein, Y., Herut, B.,
Ozer, T., Zlatkin, O., Bar, T., et al. (2024).
Temporal pattern and profile of a coastal‐
deep sea conveyor at a marginal deep
oligotrophic sea. Journal of Geophysical
Research: Oceans,129, e2023JC020441.
https://doi.org/10.1029/2023JC020441
Received 4 SEP 2023
Accepted 19 APR 2024
Author Contributions:
Conceptualization: Ronen Alkalay,
Yishai Weinstein, Timor Katz
Formal analysis: Timor Katz
Investigation: Ronen Alkalay,
Yishai Weinstein, Barak Herut, Tal Ozer,
Olga Zlatkin, Tslil Bar, Timor Katz
Methodology: Barak Herut
Supervision: Yishai Weinstein,
Barak Herut, Tslil Bar, Ilana Berman‐
Frank, Timor Katz
Writing – original draft: Ronen Alkalay
© 2024. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution‐NonCommercial‐NoDerivs
License, which permits use and
distribution in any medium, provided the
original work is properly cited, the use is
non‐commercial and no modifications or
adaptations are made.
ALKALAY ET AL. 1 of 19
the marine carbon cycle in enclosed, marginal seas like the eastern Mediterranean, where the ratio of coastal
length to basin area is very large, was hardly studied (e.g., Karageorgis et al., 2008).
The margins‐deep sea transfer of PM is commonly facilitated by surface and intermediate nepheloid layers (SNL
and INL), which owe their existence to variable factors, such as riverine input, biological processes, resuspension
by waves and currents on the shelf and internal waves and submarine canyons on the slope (Cacchione &
Drake, 1986; Dickson & McCave, 1986; McCave, 2009; McPhee‐Shaw, 2006; McPhee‐Shaw et al., 2021; Özsoy
et al., 1989; Tian et al., 2022).
This study focuses on the southeastern Levantine Basin (LB), at the eastern margin of the Eastern Mediterranean
Sea (EMS). This is a stratified, highly oligotrophic, warm water body with extremely low primary production
(32 g C m
2
yr
1
; Rahav et al., 2013; Reich et al., 2022), and where the deep basin (1.5–2 km) is very close (30–
40 km) to the shore. The main current flow and sediment transport direction next to the basin margins is along‐
shore (e.g., Rosentraub & Brenner, 2007). Nevertheless, recent sediment trap study suggested that the main
source of POC to the deep southeastern LB is the nearby margin via lateral transport, probably related to
ephemeral river discharge and resuspension on the narrow shelf (Alkalay et al., 2020). However, the spatial and
temporal patterns and the actual factors that control this lateral transport and the resulting export in the deep basin
remains un‐characterized. The relation of this cross‐shore transport to the turbidity currents described in small
submarine canyons east of the study area (Jaijel et al., 2023) should also be considered.
We hereby analyze a 4‐year data set from sediment traps, along with
234
Th and POC profiles from the deep LB, in
order to gain insights into the driving mechanisms of the coastal‐deep sea conveyor, which predominates POC
export in the EMS deep basin.
2. The Levantine Basin and Study Site
2.1. The EMS and LB
The Mediterranean Sea is a semi‐enclosed basin characterized by limited water exchange with the Atlantic Ocean
through the Gibraltar Strait. While the general flow pattern of the surface water is eastwards, in the eastern basin it
follows a cyclonic along‐slope pattern. In the LB, the surface water splits into the Mid‐Mediterranean Jet, which
meanders through the center of the basin and the cyclonic current that flows along slope (Brenner, 2003; Hecht
et al., 1988; Malanotte‐Rizzoli et al., 2014; Robinson et al., 1992). The circulation over the Israeli shelf is
characterized with strong seasonal variability, reaching maximal velocities in winter and summer. During winter
storms, strong southwesterly winds drive a northward current, which can induce downwelling over the shelf and
the intensification of deeper level currents near the shelf break, with a potential to intense shelf to open sea
transport (Rosentraub & Brenner, 2007). Levantine Intermediate Water (LIW) formation occurs in winter as
surface water cools down and sinks along isopycnals to intermediate depths (Lascaratos et al., 1999). The LIW
flows westwards across the entire basin spreading from the Levantine into the Ionian basin and through the Sicily
Straits into the western Mediterranean (Malanotte‐Rizzoli et al., 2014; Robinson et al., 2008; Tanhua et al., 2013).
The LB anti‐estuarine flow regime results in extremely oligotrophic conditions (Reich et al., 2022; Siokou‐
Frangou et al., 2010; Tanhua et al., 2013), accompanied by high salinity (average of ∼39‰) and a relatively
warm sea. The relatively high surface temperature (17–27°C), causes water column stratification, which reduces
winter mixing, therefore inhibits nutrient import from the deep water into the euphotic zone, adding to the low
productivity in this basin (Azov, 1991; Hazan et al., 2018; Ozer et al., 2022; Taucher & Oschlies, 2011). The
Israeli coastline of the LB displays a generally even topography, transitioning gradually from a northeasterly
direction in the south to a northerly orientation in the north (Figure 1). In the northern region, the shelf measures
10–15 km in width, featuring a steep slope ranging from 8 to 10°. This slope is intricately carved by small
submarine canyons, plunging to depths exceeding 1,000 m within 30–35 km from the shore. Heading south of
Haifa, the shelf widens to 15–20 km, and the slope becomes more moderate, reaching 2 degrees offshore Gaza
(Almagor, 1993; Ben‐Avraham et al., 2006).
2.2. Study Site
The field work was conducted at two moorings sites in the deep basin of the southeastern LB, DeepLev (32.9995/
34.4997; T. Katz et al., 2020) and THEMO (32.8000/34.3861; Ben Ezra et al., 2021; Reich et al., 2022; Figure 1).
The sites are located 20 km apart at 1,500 m depth, ca. 50 km northwest‐west of Haifa.
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ALKALAY ET AL. 2 of 19
The continental shelf east of the study area is merely 10–15 km wide and the slope is steep (10°), dissected by
submarine canyons (Almagor, 1993; Ben‐Avraham et al., 2006; O. Katz, 2012). The nearby coastal hydrography
(northern Israel) is characterized by small streams draining low‐lying areas (below 1,000 m above sea level), with
drainage areas varying from tens to hundreds of square kilometers, reaching up to 1,100 square kilometers in the
case of the Qishon River (Vachtman et al., 2013) (Figure 1).
3. Methods
3.1. Field Work
Water column profiles were sampled for POC (4–5 L) and
234
Th (2–4 L) during 10 visits to DeepLev and
THEMO sites (Figure 1) between 2017 and 2020. Profiles included 9–13 samples, down to 1,300 m (200 m above
seafloor), which were taken using either a 12 or 24‐Niskin bottle rosette onboard the R/V Bat Galim or the R/V
Med‐Explorer. In April 2018, we also sampled along a cross‐shore transect en route to DeepLev, at water depths
of 120, 860 and 1,200 m (t1, t2, t3, Figure 1), including 5–8 sampling depths. Sinking particles were sampled by
three automatic McLane sediment traps (McLane Parflux 21 cup; 0.5 m
2
surface area), positioned on a bottom‐
tethered mooring at the DeepLev site during December 2016 through February 2020. The traps were located at the
base of the euphotic zone (180 m), 100 m below (280 m) and 200 m above seafloor (1,300 m). Sampling intervals
were set to 11–12 days per sample. Further details can be found in T. Katz et al. (2020).
3.2. Sample Processing and Analyses
Thorium in the bulk sample was co‐precipitated with potassium permanganate (e.g., Benitez‐Nelson, Buesseler,
Rutgers van der Loeff, et al., 2001; Buesseler et al., 2001; Rutgers van der Loeff et al., 2006) within 1–2 days of
sampling. When not performed immediately after sampling, samples were pre‐acidified onboard with concen-
trated HNO
3
to a pH of <2.0, and then buffered to pH of 8.0–8.2 by adding an NH
4
OH buffer solution prior to co‐
precipitation. The 2018 samples were spiked with
230
Th (10 dpm), which was used as a yield monitor (Pike
et al., 2005). Samples were left overnight and then vacuum‐filtered through 47 mm quartz microfiber (QMA)
Figure 1. Stations location and map of the study site, including the DeepLev (DL) and THEMO moorings, cross section of the
DeepLev transect (t1 to DL, dotted line), Hadera Gloss#80 station (2.2 km offshore) and depths of the DL sediment traps
(yellow triangles).
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filters (nominal pore size 1 μm) and dried at 50°C. Filter punches (diameter of 19 mm), covered with Mylar film
and double layer of Al foil (Benitez‐Nelson, Buesseler, Rutgers van der Loeff, et al., 2001; K O Buesseler
et al., 2001; P. Cai et al., 2006) were counted using an anti‐coincidence, 5 sample, low‐level Risø beta counter.
Samples were then allowed to decay for at least six
234
Th half‐lives (144 days) before a final beta count was taken
in order to determine background radioactivity (Buesseler et al., 2001; Pike et al., 2005). The beta counter was
calibrated against IRMM
238
U standard, which was spiked onto filters covered by MnO
2
precipitates. After final
counting, the MnO
2
precipitates were dissolved by HNO
3
+HF, and thorium was pre‐concentrated, using an
AG1‐X8 resin‐filled column (Pike et al., 2005).
230
Th was then analyzed by an NU Instrument MC‐ICPMS in the
Geological Survey of Israel, using a
229
Th spike as a yield monitor. Calculated thorium yields averaged 98%, with
a few samples <90%, which were corrected accordingly. 100% efficiency was assumed for all non‐spiked samples
(4 profiles from 2019 to 2020).
Particulate mass flux (PMF) was measured as the dry weight of salt‐rinsed sediments in the collection bottles of
the sediment traps. PM from each bottle was run through 1 mm mesh sieve to separate large particles and
zooplankton, and then split into 10 equal aliquots, using a WSD‐10 wet sample divider. After drying, five
subsamples were weighed separately using analytical scales (measured mass replication error <2%), and their
average weight was used for PMF calculation.
Samples for POC (filtered through a GF/F 25 mm membranes or dried material from the traps) were placed in tin
or silver cups, fumed by hydrochloric acid (HCl 32%) to remove inorganic carbon, and analyzed by a Perkin
Elmer 2400 CHN Elemental Analyzer for POC. Common organic carbon standards, such as Caffeine, L‐glutamic
acid, Glycine, and Nicotinamide were used for CHN calibration.
Current measurements were performed by three downward looking ADCPs (Acoustic Doppler Current Profilers)
and one Aquadopp current meter, which are integral components of the DeepLev mooring station (T. Katz
et al., 2020). The current meters include an RDI WorkHorse 300 kHz located at 30 m, covering the depth range of
∼30–90 m with a vertical resolution of 2 m, and a temporal resolution of 15 min and two RDI QuarterMasters
150 kHz, which were deployed at 100 and 375 m, covering the depth range of ∼100–700 m, with a vertical
resolution of 10 m and a temporal resolution of 1 hr. The ADCPs went through compass calibration to ensure
angle accuracy (error kept under 5°) and performance checks following the manufacturer recommendation and
best practice standards. The Aquadopp current meter (Nortek) was deployed at 1,315 m. Acoustic backscattering
is a reliable proxy for turbidity, that is, suspended particle concentration (e.g., Chanson et al., 2008; Thevenot
et al., 1992; Thorne & Hurther, 2014). In this study, we used the Acoustic backscattering intensity (ABI)
measured by the Aquadopp current meter at 1,315 m between November 2016 and September 2020 (excluding
some pauses during recovery and redeployment of the mooring) with resolution of 30 min. The ABI was not
calibrated to particle concentration; therefore, measurements are presented as counts, assuming positive corre-
lation with concentration as presented in Figure 7.
4. Results
4.1. Total Mass and POC Fluxes in Traps
The particulate matter collected by the traps is dominated by lithogenic material (clay to silt grain size, see also T.
Katz et al., 2020), adjoined by plankton shells (formainifera, diatoms, radiolaria, see Avnaim‐Katav et al., 2020)
and organic matter, which mainly consists of fecal pellets of variable size and decaying soft issues (gel‐like
material).
Total mass fluxes (TMF) and POC fluxes documented by the DeepLev sediment traps are presented in Figures 2a
and 2b. POC flux is strongly correlated with the TMF pattern in the deep traps (e.g., r=0.947 and 0.772 at the
1,300 and 280 m traps, respectively, Table 1), while less at the base of euphotic zone (180 m, r=0.328). Whereas
there is a good correlation between shallow and deep fluxes (Table 1), both TMF and POC fluxes were: (a) much
higher during the rainy season, characterized by strong peaks during and following winter storm events and (b)
higher at the deep water as compared with the shallow (180 m) trap (mainly during the winter; Figure 2b). The
POC fraction was larger at the shallow trap (14.8% and 3.5% for the 180 and 1,300 m, respectively), as well as
during the summer compared with winter events (e.g., 22.3% and 4.5%, at 180 m and 5.2% and 2.1% at 1,300 m,
respectively). Average winter fluxes of TMF and POC are presented for 2018 and 2019 in Figure 3a. While fluxes
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ALKALAY ET AL. 4 of 19
were significantly higher in 2019 (except for POC at 180 m, which was slightly higher in 2018), the pattern of
larger fluxes at depth and the POC‐TMF correlation were maintained in both years. POC fractions were quite
similar during both winters, although in the 180 m trap the POC percentage was higher in 2018 (6.0% and 4.0%,
respectively). During the winter of 2020, while TMF increased as well (Figure 2a; yet unfortunately, there is no
POC data for 2020), the peak was much smaller than in both 2018 and 2019.
Figure 2. Time series at the DeepLev mooring: (a) total mass (TM) and (b) particle organic carbon (POC) fluxes in DeepLev
as measured 50 km south of Haifa; (c) acoustic backscatter intensity (ABI), measured by an Aquadop acoustic current‐meter
positioned at 1,310 m at DeepLev; (d) significant wave height (above 1 m), at the Hadera Gloss#80 station, run by IOLR and
(e) discharge of the Qishon River data (obtained from the Hydrological Service of Israel).
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Table 1
Pearson Correlation Coefficients Between Different Parameters Measured at the DeepLev Mooring and the Nearby Coastal
Area (See Text for Explanations)
Parameters Time lag r n p Period
TMF versus POC (1,300 m) No lag 0.947 83 <0.01 November 16–December 19
No lag 0.967 51 <0.01 December 17–December 19
ABI versus TMF (1,300 m) No lag 0.718 84 <0.01 November 16–March 20
TMF: 1,300 versus 280 No lag 0.766 70 <0.01 December 17–September 20
11 days lag 0.538 68 <0.01
22 days lag 0.257 66 <0.05
TMF: 1,300 versus 180 No lag 0.553 70 <0.01 December 17–September 20
11 days lag 0.541 68 <0.01
22 days lag 0.282 66 <0.05
Qishon versus 1,300 m trap No lag 0.241 28 0.22 December 17–March 20
11 days lag 0.468 26 <0.05
22 days lag 0.559 24 <0.01
33 days lag 0.154 22 0.49
Qishon versus 280 m trap No lag 0.123 28 NC December 17–March 20
11 days lag 0.463 26 <0.05
22 days lag 0.089 24 NC
33 days lag 0.070 22 NC
Qishon versus 180 m trap No lag 0.173 28 0.38 December 17–March 20
11 days lag 0.806 26 <0.01
22 days lag 0.320 24 0.13
Wave versus 1,300 m trap No lag 0.254 86 <0.05 November 16–March 20
11 days lag 0.499 85 <0.01
22 days lag 0.573 84 <0.01
33 days lag 0.606 83 <0.01
44 days lag 0.626 82 <0.01
55 days lag 0.443 81 <0.01
Wave versus 280 m trap No lag 0.057 55 NC December 17–March 20
11 days lag 0.501 54 <0.01
22 days lag 0.545 53 <0.01
33 days lag 0.392 52 <0.01
44 days lag 0.352 51 <0.05
Wave versus 180 m trap No lag 0.196 53 0.16 December 17–March 20
11 days lag 0.471 52 <0.01
22 days lag 0.366 51 <0.01
33 days lag 0.430 50 <0.01
Qishon versus ABI No lag 0.562 113 <0.01
33 days lag 0.384 107 <0.01
66 days lag 0.307 101 <0.01
180 m current versus 180 m trap Winter only
(December–May)
0.039 26 NC December 17–March 20
280 m current versus 280 m trap 0.508 22 <0.05 December 17–March 20
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ALKALAY ET AL. 6 of 19
4.2. Water Column POC and
234
Th
Water column profiles of POC are shown in Figure 4a, including some profiles from Alkalay et al. (2020)
(December 2017, April and June 2018). In general, POC was relatively high (mainly, 20–40, and up to 53 μg l
1
)
in the euphotic zone (upper 180 m) and usually showed a gradual decrease between surface water and 280 m
(100 m below the base of the euphotic zone). At deeper water, concentrations were mostly ≤10 μg l
1
, although
higher concentrations were observed during February–March 2018 and December 2017 (20–30 μg l
1
). A peak of
20–25 μg l
1
was observed at 700 m in May–June 2018 and February 2020. Next to the Seafloor (1,500 m), POC
was relatively high (25–35 μg l
1
), whenever measured. POC concentrations measured along the DeepLev
Figure 3. Total mass flux (TMF) and particulate organic carbon (POC) fluxes measured by the DeepLev McLane traps during
the winters of 2017–2018 and 2018–2019 (mid‐December through mid‐March; DeepLev was off duty between 20.3 and 2.5,
2019); (a) daily average flux; (b) the increase in flux per meter depth ((e.g., {flux
(280)
flux
(180)
}/100 m)).
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ALKALAY ET AL. 7 of 19
transect (10/4/2018; Figure 1) were higher at the shelf break (56–32 μg l
1
, Figure 4b,t1), while similar to
DeepLev concentrations at all other offshore stations (t2–t3, Figure 1).
In Figure 5, we show
234
Th profiles taken between December 2017 and February 2020, including three that were
already presented in Alkalay et al. (2020).
234
Th activities are compared with the activities of its radioactive
parent
238
U, which was determined as 2.56 (±0.07) dpm l
1
for the LB (Alkalay et al., 2020). When in equi-
librium, daughter and parent activities (A) are expected to be identical. Accordingly, deficit is identified when
234
Th
A
<
238
U
A
, while excess is reserved for cases where
234
Th
A
>
238
U
A
.
234
Th in the euphotic zone (above
180 m) was commonly in disequilibrium with
238
U during the winter (December 2017 and 2019, February
through April 2018 and February 2020, Figure 5a). This was mostly expressed as a
234
Th deficit, as is usually the
Figure 4. Depth‐profiles of particulate organic carbon (POC) concentrations (a) at DeepLev and THEMO stations (the latter marked with *) between February 2018 and
February 2020, and (b) along the Haifa‐DeepLev transect (10.4.2018; locations in Figure 1).
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ALKALAY ET AL. 8 of 19
case in other oceanic settings (Benitez‐Nelson, Buesseler, Karl, & Andrews, 2001; Buesseler et al., 2009;
Planchon et al., 2013), although in March 2018
234
Th excess was also observed at certain euphotic zone depths
(Figure 5a). Euphotic zone
234
Th deficit was also observed along the Haifa—DeepLev transect in April 2018
(except for the 1,200 m depth station, Figure 5b). Importantly, winter disequilibrium was also observed in
DeepLev deep water. This was mostly expressed as an excess during February through April 2018 (with a very
prominent excess in March, Figure 5a) and in December 2019 (as high as 5 dpm l
1
at 700, 900 m), although a
large deficit was observed in December 2017. On the other hand, near‐equilibrium activities were observed in
March 2019 and February 2020. During the summer (May through August), activities clustered around equi-
librium values (Figure 5a), although in mid‐May 2019 there was a clear deficit in the euphotic zone, as well as at
depth.
The typical high POC, combined with the
234
Th‐deficits in surface water results in a decreasing gradient of
POC/
234
Th ratios from surface water toward the base of the euphotic zone (30–13 and 3–10 μg dpm
1
, Figure 6).
Ratios remain steady throughout the twilight zone and the deep water (3–13 μg dpm
1
) excluding next to the
seafloor, where the increase in ratios (13 μg dpm
1
) can be attributed to resuspension of seafloor sediments
(Alkalay et al., 2020). Notably, a positive POC/Th anomaly was commonly observed at 700 m depth.
Figure 5.
234
Th depth profile (a) at DeepLev and THEMO stations, sampled between February 2018 and February 2020; and (b) along the Haifa‐DeepLev transect
(Figure 1). The reference
238
U activity (dotted line) is from Alkalay et al. (2020).
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4.3. Turbidity
Acoustic backscatter intensity (ABI), measured on the DeepLev mooring at
1,315 m, is used here as a proxy of turbidity. ABI was low and steady at 60–65
counts during summer, while peaks of up to 95 counts were measured during
the winter period, which was followed by a slow decline (3–4 months) to
summer low levels (Figure 2c). Importantly, the ABI shows a good correla-
tion with TMF and POC collected in the adjacent 1,300 m sediment trap
(Pearson correlation coefficient of r=0.72, p<0.001, Table 1; also see
Figures 2a–2c and 7).
4.4. Current Velocities
In Figure 8we show current rose‐diagrams and time series of the E‐W ve-
locity component (cross‐shore) at selected depths, measured by ADCPs at
DeepLev (50 km offshore) over the period of 2017–2021. While the main
flow was to the north‐northeast (Figure 8a), westward off‐shore flow (i.e., a
negative cross‐shore component) was recorded in 44%–52% of our readings
for the presented depths. Importantly, when westward flow occurred, it ten-
ded to be predominant (2–4 times higher than the long‐shore component),
although this relation was not necessarily maintained during winters.
Nevertheless, during the winter, westward currents often reached >10 cm s
1
, with peaks of up to 60 cm s
1
(50 km d
1
) at 50 m and 20 cm s
1
(17 km d
1
) at 280 and 420 m (Figure 8b). Averages of westward flow during
the winters (January–April) of 2019–2021 were significantly higher than that during 2018 winter at all depths
(except for the 2020 season at 100 m; Figures 8b and 8c). For example, at 180 m, the 2018 winter average was
0.95 cm s
1
, compared with 1.7–3.5 cm s
1
during the winters 2019 through 2021 (in 2019 data covers only
January–March, and in 2020 it is mid‐February to April). Nevertheless, peak flows during the winter of 2020 were
lower than both 2019 and 2018 (Figure 8c), especially at 100 and 180 m.
5. Discussion
5.1. Depth of the Lateral Conveyor
The data set from the DeepLev sediment traps displays several important observations. First, TMF and POC
fluxes are higher during the winter, characterized by strong peaks. Second, fluxes largely increase with depth
(Figures 2a, 2b, and 3a). Both observations have been attributed to the dominance of a lateral PM, detrital input
from the nearby coast or shelf, which mainly operates during the winter (Alkalay et al., 2020). Third, the patterns
of both TMF and POC fluxes at the shallow trap (180 m, base of euphotic zone) correlate positively with fluxes at
the deeper traps, in particular during the winter (Figures 2a and 2b;r=0.55 (p<0.01) for TMF at 180 and
1,300 m, Table 1). Since fluxes at depth are evidently controlled by lateral influx, this suggests that the lateral
conveyor is a major export factor also at the base of the euphotic zone. Another interesting observation is that
TMF and POC fluxes at all depths were much higher during the winter of 2019 than during 2018 (excluding POC
at 180 m, Figures 2a and 3a), which will be discussed in Section 5.3.
Investigation of the increase in flux with depth during the winter (Figure 3b)
reveals that the predominant lateral PM influx occurred at relatively shallow
depths. Specifically, TMF flux at 280 m was 388 and 511 mg m
2
d
1
for the
winters of 2018 and 2019, respectively, which is 0.66 and 0.51, respectively,
of that observed at 1,300 m during the same period. Assuming a unimodal
lateral influx, and that a major part of the winter 180 m flux is also lateral,
most of the flux documented at 1,300 m is probably due to lateral influx at no
deeper than 400–500 m. Similar patterns of increased flux with depth were
reported in other margins (e.g., the Eel River continental slope; Walsh &
Nittrouer, 1999), which was also attributed to the combined effect of shelf
resuspension, river discharge, and margin circulation. However, in the LB
case, the observation is from the deep basin, and it clearly suggests that the
assumingly shelf‐derived detrital conveyor keeps a shallow trajectory.
Figure 6. POC/
234
Th ratios measured at the DeepLev and THEMO sites.
Figure 7. Total mass (TMS) at 1,300 m depth compared with the acoustic
backscatter intensity (ABI), measured by the Aquadop current meter at
1,315 m.
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Figure 9shows a
234
Th‐based POC cumulative flux along the water column at DeepLev. As already pointed out
by Alkalay et al. (2020), the repeated large disequilibrium at depth, especially during winter, which is not
balanced by export from the euphotic zone, should reflect on the lateral influx from the margins. The pattern
portrayed in Figure 9highlights several important features of the Th profiles. First, the predominance of Th
excesses at depth (Figure 5) during winter, which is presented as a “negative export” in Figure 9, as well as the less
common deficits (e.g., December 2017), designated in the figure as positive export. Second, when disequilibrium
was observed, it often occurred along the entire water column, down to 1,300 m (see also Figure 5). Third, most
disequilibrium profiles were observed in the 2018 winter, while much less in 2019 and 2020, although this could
be related to the much fewer profiles taken during 2019–2020. The 2018–2019 differences will be discussed
separately in Section 5.3.
Thorium budget in deep water includes (a)
238
U decay (source), (b) possible lateral influx (source), (c) vertical
influx (source), (d) vertical (possibly lateral) outflux (sink) and (e)
234
Th decay (sink). Since
238
U decay is
constant, disequilibrium is mainly attributed to imbalance between influx (vertical and lateral) and out‐flux.
Accordingly, when
234
Th deficit is observed (e.g., December 2017, Figure 5), outflux is larger than the influx,
while when an excess is observed (e.g., February and March 2018), the influx (mainly lateral) is larger than the
outflux. In open oceanic settings, where influx and outflux are assumed solely vertical, excess at depth is
attributed to remineralization of sinking PM (e.g., Owens et al., 2014), which implies that the integrated excess
below the euphotic zone is either equal or smaller than the deficit at the euphotic zone. Apparently, the deep water
disequilibria (excesses and deficits) observed in our study (Figure 9) are not balanced by the euphotic zone
deficits, therefore they should be related this way or another to the lateral detrital influx.
Importantly, these disequilibria were often observed down to 1,300 m (Figures 5and 9), which suggests that
significant influx also occurs in deep water, apparently in disagreement with the picture portrayed by the sediment
traps observations (flux no deeper than 500 m, see above). We suggest that this is the result of the operation of a
double‐headed conveyor, as follows. The material collected by the traps is composed of a relatively coarse
sediment fraction (note that by “coarse” we mean silt and clay, T. Katz et al., 2020), transported at shallow to mid
water‐depths (100–500 m), while that documented also at greater depths by the thorium is mainly carried by a
Figure 8. (a) Rose diagram of current velocity and direction at different depths, measured on the DeepLev mooring during 2017–2021 (percentage is of total data); (b) E–
W component of (cross‐shore) current velocity at different depths, measured on the DeepLev mooring; (c) box plot of winter westward current velocity during 2018–
2020 (“x” stands for the average).
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ALKALAY ET AL. 11 of 19
finer fraction (colloid‐dominated, e.g. Baskaran et al., 1992; Degueldre & Kline, 2007; Lin et al., 2014; Santschi
et al., 2006). Both could be envisaged as intermediate (INL), but while the first originates from the shelf, the latter
could as well be associated with the continental slope processes (e.g., Cacchione & Drake, 1986; Dickson &
McCave, 1986; Gardner & Walsh, 1990; Pak et al., 1978). Recently, Jaijel et al. (2023) documented turbidity
Figure 8. (Continued)
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ALKALAY ET AL. 12 of 19
currents that transport material >830 m through a canyon crossing the continental slope, 30 km east‐southeast of
DeepLev. While the coarse‐grained material carried by these currents evidently deposited inside the canyon, the
currents and follow‐up processes (wash and turbulence) could launch the finer material to the deep basin along
isopycnal surfaces, where the DeepLev mooring is located (Jaijel et al., 2023). Similar observations, of INL
associated with canyons have been described elsewhere (e.g., Gardner, 1989; Hickey et al., 1986; McPhee‐Shaw
et al., 2021; Puig & Palanques, 1998), thus warranting a more detailed investigation of the relationships between
slope processes and INLs in the deep LB.
5.2. Source, Trajectory and Time Scales of the Shallow INL
The source for the shallow INL could be either land derived flow of PM or resuspension on the continental shelf
(or both). Alkalay et al. (2020) already showed that the input is event‐related, portraying a good positive cor-
relation with both wave height on the shelf and with coastal rain and stream discharge (Figures 2d and 2e). In
Table 1we show Pearson correlations between coastal processes and fluxes in the traps. All traps show good
correlation between stream discharge (Qishon River next to Haifa, see Figure 1, which is taken as representative
of northern Israel coastal streams) and TMF (r=0.46–0.81, p<0.05 for the winters of 2018–2020), with a lag of
10–20 days (i.e., 1–2 trap bottles, 20 days is for the deep, 1,300 m trap). TMF also shows a good correlation with
wave height at the shelf (r=0.47–0.63, p<0.01, Table 1), although with significantly larger lags in the deeper
traps (20–40 days), which should be further tested and studied. Accordingly, it is concluded that the shallow INL
is nourished by coastal and shelf processes, mainly during the winter.
The input depth of 100–500 m at the DeepLev site suggests that this INL keeps a shallow trajectory, with minimal
deepening at 30–35 km from the shelf break. With the particles leaving the shelf at depths of 0–150 m and
reaching the DeepLev site at 100–500 m, and a travel time on the order of 10 days (based on the stream discharge‐
shallow trap lag, Table 1), settling velocities should be no larger than 50 m d
1
. This is significantly lower than the
200–400 m d
1
calculated by some authors (Armstrong et al., 2009; Buesseler et al., 2007; Xue & Arm-
strong, 2009), but in agreement with the moderate rates of <100 m d
1
calculated for the depth range of 150–
500 m, based on
210
Po‐
210
Pb disequilibrium measurements (Ceballos‐Romero et al., 2016; Villa‐Alfageme
et al., 2014). Also, the relatively small particle size (silt to clay, T. Katz et al., 2020) could result in lower
settling rates by up to one order of magnitude (e.g., Alonso‐González et al., 2010; McDonnell & Buesseler, 2010;
Figure 8. (Continued)
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ALKALAY ET AL. 13 of 19
Stemmann et al., 2004). The relatively slow settling also explains the broader
winter peaks observed in the 1,300 m trap, compared with 280 m (Figure 2a),
considering that most of the particles observed at 1,300 m arrive vertically
from 100 to 500 m.
While the PM observed in the traps is predominantly supplied by flash floods
and resuspension by shelf waves, cross‐shore flow is also required in order to
transport the PM to the deep basin. At the EMS, mid‐depth water hosts the so‐
called LIW (Levantine Intermediate Water) water mass (150–350 m, Hazan
et al., 2018; Ozer et al., 2017), which is indeed believed to derive from the
margins (Fach et al., 2021; Menna & Poulain, 2010); however, its temporal
pattern and dynamics are hardly known. Current velocities measured during
the present study, although unfortunately limited only to the DeepLev site,
50 km offshore, clearly show that while average westward velocities are
relatively low (1.7 and 1.5 km d
1
at 100 and 280 m, respectively), there are
several velocity peaks (up to 40 and 17 km d
1
, respectively, Figures 8b and
8c), which could allow the arrival of PM at DeepLev within several days, in
agreement with the above. Specifically, the large peak in fluxes observed in
the traps during the beginning of February 2018 (Figure 2a) follows up on a
large westward current velocity peak, which centered on January 29‐30
(Figure 8b). In 2019, a wide flux peak during January (Figure 2a) followed
several current velocity peaks, which commenced December 29–30 and
continued till the end of January (Figure 8b). Notably, the apparent inter‐
annual difference in westward velocities (Figures 8b and 8c), together with
higher discharge in coastal streams, could be the reason for the larger TMF
and POC fluxes observed during 2019, compared with those documented in
2018 (Figures 2a and 3a), which is further discussed below. Similarly, the
limited high‐velocity peaks in the winter of 2020 (Figure 8c) could explain the
relatively low fluxes measured in traps during this year (Figure 2a). We note
that the good positive correlation between terrestrial discharge and turbidity
(ABI) at 1,300 m (r=0.72; p<0.01, Table 1) suggests that the deep INL (i.e.,
the finer‐PM influx, suggested by the
234
Th profiles) is also linked to coastal
processes. This could be an offshoot of density currents traveling through
slope submarine canyons during winter events (Jaijel et al., 2023).
5.3. Transport‐Export Budgets
As resolved earlier, sediment traps and the
234
Th‐calculated POC fluxes present decoupled INL patterns, with the
first attesting to a shallow to middepth lateral influx and the latter indicating a deep input as well, down to at least
1,300 m (compare Figures 2a, 2b, 3, 5, and 9). Moreover, while the traps showed higher fluxes during the 2019
winter (Figures 2a, 2b, and 3), the thorium seemingly demonstrated greater inputs or outputs (excesses and
deficits, respectively) during the 2018 winter (Figure 9, also summarized in Table 2). Unfortunately, the 2019
Figure 9. POC cumulative fluxes, calculated from
234
Th profiles (Figure 5)
and POC/
234
Th ratios (Figure 6). The calculations assume a linear change in
234
Th concentration between sampling depths. Red is for 2018 (starting
December 2017), purple is for 2019 and green is for 2020 (starting
December 2019).
Table 2
2018 and 2019 Winter (3 Months) POC Flux From the Deep (1,300 m) Trap and That Calculated From
234
Th Fluxes (Figure 9) and POC/234
Th
Ratios at DeepLev
(Figure 5)
Period McLane trap =output (mmole C m
2
)
234
Th‐calculated =output ‐ input (mmole C m
2
) Apparent input
a
(mmole C m
2
)
16.12.17–15.1.18 8.9 ±0.4 589 ±192 0
15.1.18–14.2.18 35.4 ±1.9 385 ±142 420 ±142
14.2.18–15.3.18 24.9 ±1.0 667 ±237 692 ±237
17.12.18–16.3.19 111.3 ±2.5 100 ±151 211 ±151
Note. 2018 is presented as three periods, owing to three
234
Th profiles taken during this period, while 2019 is presented as one continuous period with only one
234
Th
profile taken in March 2019.
a
Apparent input =measured trap POC flux minus
234
Th‐calculated POC flux; note that input cannot be negative (e.g., December 2017).
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ALKALAY ET AL. 14 of 19
winter season is represented only by one profile from March, but considering the relatively long life of
234
Th
(mean life of 34.8 days), this equilibrium could be taken as a rough representation of the entire winter season.
A possible explanation for the trap‐
234
Th decoupling between the 2018–2019 winters could be that it reflects on
the extent of downward outflux in the two seasons. That is to say that while in 2019 there is evidently (traps) a
larger vertical export, which could balance the
234
Th (and POC) input by the lateral conveyor, the smaller vertical
export observed in traps during 2018 resulted in
234
Th excess, that is, lateral influx was not balanced by vertical
outflux. This is also apparently supported by the relatively high deep POC observed in February and March 2018
(Figure 4a), which supposedly suggests that imported POC was not efficiently removed by export processes.
However, budgets do not conform to this reasoning (Table 2). As shown in Table 2, apparent input equals output
measured by the traps minus
234
Th‐calculated POC flux (except for December 2017, where input is conserva-
tively assigned zero, see comment in Table 2). Accordingly, at 1,300 m, the 2018 winter season (mid‐December to
mid‐March) apparent input summarized as 1.1 mol POC m
2
, while that calculated for the 2019 winter was much
lower (0.2 mol POC m
2
). We note that core top observations at the DeepLev site indicate that the average POC
flux to seafloor (1,500 m depth) is 0.13 mol m
2
yr
1
(T. Katz et al., 2020), which nicely agrees with the 2019
1,300 m trap winter observations (when most export occurs, see Figure 2a) and suggest that the extra input
calculated for 2018 does not commonly arrive at the seafloor in this area.
Alternatively, we suggest that the discordance between the 2018 and 2019 winters should be comprehended in
terms of discharge and transport variability. In 2018, discharge peaks were relatively low and ended in mid‐
February (Figure 2d), associated with a sharp decline in fluxes in the traps (Figures 2a and 2b). As a result,
the fine‐grained PM (colloids), which as suggested above was probably related to slope processes, could “sur-
vive” in the water column, leading to the build‐up of excess
234
Th during February through April (Figures 5a and
9). On the other hand, in winter 2019 discharge peaks persisted until the end of March, aided by significantly
stronger westward currents than in 2018 (e.g., average at 280 m of 1.7 and 1.2 and maxima of 17.4 and
10.4 km d
1
in 2019 and 2018, respectively). This resulted in higher vertical fluxes documented by the traps
(Figures 2b and 2d), accompanied by the removal of the
234
Th‐loaded fine‐grained PM, which was reflected in the
March 2019 profile in close to equilibrium values of
234
Th (Figures 5a and 9). Thus, the values in Table 2are
underestimating the input to the deep basin, in particular for the fine fraction in 2019. This is also reflected in the
turbidity (ABI) at 1,300 m, which sharply declined in March 2019, probably due to the insinuated removal of the
fine‐grained PM (Figure 2c; note that the build‐up of a small peak in the beginning of May 2019 is associated with
a deep‐water
234
Th deficit during the same time, Figure 5a).
The deep deficit observed in the 4 December 2017 profile (beginning of the 2018 winter season, Figures 5and 9)
demands a short discussion. The large deficit observed, mainly at 1,300 m (Figures 5and 9) is not backed up by
fluxes in the deep trap at DeepLev (also during the preceding month, Figure 2a and Table 2), so removal by large
vertical fluxes cannot be insinuated. Unless analytical, the large deficit probably demonstrates a non‐sinking
scavenging “event,” namely: scavenging by suspended fine‐grained (colloidal) PM that continued traveling
(laterally) farther away, through the basin, which should be further studied. We note that the relatively high POC
in deep water during December 2017 (Figure 4a) favors a significant input, but in Table 2we conservatively
assumed a zero input for the period of mid‐December 2017 through mid‐January 2018.
5.4. POC Export in Marginal Seas
We showed in this paper that although river discharge to the southeastern LB is relatively limited, coastal flash
floods and shelf waves, as well as cross‐shore currents are predominant in the POC budget and export of the deep
basin. This is unlike observations from the NW Mediterranean Sea (Barcelona margin), where suspended particle
fluxes did not show a clear relationship with either riverine discharge or wave storm events (Puig et al., 2000).
Nevertheless, this implies that the a‐priori use of primary production data for POC export modeling (e.g.,
Guyennon et al., 2015) needs to be further assessed for the deep LB, which could probably be true elsewhere for
next to shore deep basins (e.g., Heussner et al., 1990; Radakovitch et al., 1999), as well as for next to slope areas of
the deep ocean (e.g., Jahnke et al., 1990). Furthermore, the prominent differences between the 2018 and 2019
winter seasons, which are apparently related to both discharge pattern and current velocity, suggest that major
changes in flash flood intensity, related to land use modification or to climate change could have a large impact on
the deep basin POC export, as well as on other attributes of interest (e.g., nutrients, contaminants).
Journal of Geophysical Research: Oceans
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ALKALAY ET AL. 15 of 19
The above discussion also suggests that the lateral margin conveyor (INL) operates in multiple depths,
probably due to different processes and mechanisms. While the shallow influx is directly related to flash
floods and shelf processes, and is manipulated by cross‐shore currents, the deeper INL (finer‐grained, probably
colloid‐dominated material) could be related to other processes. These may include the dispersal and dilution
(possibly via lofting) of density currents in submarine canyons facing DeepLev after particle settling reduced
their ballast (Jaijel et al., 2023) and to post‐deposition resuspension and dispersal of these sediments by
turbulent hotspots related to internal waves along the canyons (Kunze et al., 2012; McPhee‐Shaw et al., 2021).
Yet, we note that the two INLs are not fully decoupled, which is demonstrated by the
234
Th that shows
excesses and deficits along the entire water column (Figures 5and 9) and by the relatively good correlation
between coastal discharge and turbidity at 1,300 m (Table 1). The slope and canyons' effect should be further
considered in future studies and by conducting a similar work farther south, where the continental slope is
devoid of canyons.
6. Conclusions
Our sediment trap and
234
Th data set demonstrates that POC export in the southeastern LB is controlled by the
transport of detrital material from the continental margins. The INL margin conveyor operates in a multi‐depth
pattern, including (a) shallow to middepth (∼100–500 m) INL and (b) middepth to deep (down to at least
1,300 m). The first is composed of silt to clay size PM and is related to coastal winter events and manipulated by
cross‐shore currents, while the latter is probably composed of finer‐sized PM (Colloids), which could be linked to
density currents that run through submarine canyons during winter. There is a large inter‐annual variability in
both the transport (lateral) and the export of PM and POC, which is dependent on the intensity of both coastal
processes and cross‐shore current velocity.
The interplay between the two INLs is reflected in a decoupling between traps and the
234
Th observations. Lower
intensity of storms during 2018 resulted in reduced lateral transport, therefore less vertical fluxes (export) in the
traps, which allowed the accumulation of fine‐grained PM and the observed
234
Th excesses along the water
column. On the other hand, the more intense winter of 2019, combined with higher current velocities, resulted in
larger transport, therefore greater vertical fluxes and better removal of the fine‐grained PM, which resulted in no
234
Th excess and reduced turbidity at depth.
The above suggests that changes in flash flood intensity, related either to land use modification or to the ongoing
climate change (e.g., the expected higher frequency of extreme events), could have a large impact on the land ‐
open sea exchange of carbon, nutrients and other contaminants. This could further impact the deep basin POC
export, which should be taken into account by both researchers and policy makers.
Last, this work highlights the importance of the multi‐methodological approach, in this case the combined use of
sediment traps,
234
Th profiling, turbidity and current measurements, in deciphering about both vertical export and
lateral transport in marginal basins.
Data Availability Statement
1. Data archived in a repository: Datasets for this research are available in these in‐text data citation references:
Ozer et al. (2024,https://doi.org/10.17882/98990).
2. Data published in governmental web site: Rivers discharge datasets for this research are included in this paper
are published by The Israel Water Authority (2017–2020) (https://data.gov.il/dataset/level_discharge/
resource/62cd157d‐766d‐4648‐897d‐50526f45abf9).
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We wish to express our gratitude to the
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