As part of the 2002 Shelf-Basin Interactions (SBI) process study, measurements of the seasonal variation in the export flux of particulate organic carbon (POC) are reported for the upper waters of ...the Chukchi Sea. POC fluxes were quantified by determination of
234Th/
238U disequilibrium and POC/
234Th ratios in large
(
>
53
μ
m
)
aggregates collected using in situ pumps. Samples were collected at 35 stations on two cruises, one in predominantly ice-coved conditions during the spring (May 6–June 15) and the other in predominantly open water during the summer (July 17–August 26). Enhanced particle export was observed in the shelf and slope waters, particularly within Barrow Canyon, and there was a marked increase in particle export at all stations during the summer (July–August) relative to the spring (May–June).
234Th-derived POC fluxes exhibit significant seasonal and spatial variability, averaging
2.9
±
5.3
mmol
C
m
-
2
d
-
1
(
range
=
0.031
–
22
mmol
C
m
-
2
d
-
1
)
in the spring and increasing
∼
4
-fold in the summer to an average value of
10.5
±
9.3
mmol
C
m
-
2
d
-
1
(
range
=
0.79
–
39
mmol
C
m
-
2
d
-
1
)
. The fraction of primary production exported from the upper waters increases from
∼
15
%
in the spring to
∼
32
%
in the summer. By comparison, DOC accumulation associated with net community production represented
∼
6
%
of primary production
(
∼
2
mmol
C
m
-
2
d
-
1
)
. The majority of shelf and slope stations indicate a close agreement between POC export and benthic C respiration in the spring, whereas there is an imbalance between POC export and benthic respiration in the summer. The implication is that up to
∼
20
%
of summer production
(
∼
6
±
7
mmol
C
m
-
2
d
-
1
)
may be seasonally exported off-shelf in this productive shelf/slope region of the Arctic Ocean.
The (2(1(0)Pb dating method was first introduced by Goldberg (1963), and since then has been applied to study sediment from lakes, estuaries and coastal marine environments. Hundreds of studies ...around the world have used (2(1(0)Pb as a geochronological tool in aquatic ecosystems. However little attention has been paid to the potential of this naturally occurring isotope as an environmental tracer of ecological events. Here we report three instances in which (2(1(0)Pb profiles measured on undisturbed sediment cores from lakes, rivers and fjords show us the potential of (2(1(0)Pb profile as a tracer of natural and anthropogenic processes. The methodology used here is a suite of techniques combining biogeochemistry (micro-electrodes), paleomagnetism (susceptibility), sediment characteristics (LOI) and visualization (SPI and X-ray) applied to the interpretation of (2(1(0)Pb profiles. We measured (2(1(0)Pb profiles on sediments from a river, Cruces River (Chile), which recorded a clear shift in the water chemistry caused by a pulp mill effluent to the river. Here metal mobilization and remobilization of the tracer may be the cause of the observed profile. We also measured (2(1(0)Pb profiles in sediment from two fjords of Southern Chile (Pillan and Renihue), the sudden deposition change of fresh (2(1(0)Pb with depth observed could very well be the result of bioturbation but it occurred in a seafloor area deprived of bioturbators. In this case, (2(1(0)Pb recorded the onset of aquaculture activities (fish farming) that took place two decades ago. Finally, (2(1(0)Pb profiles measured in two lakes in the ''pampa Argentina'': Epecuen and Venado showed a particular shape with depth. These profiles apparently registered a sudden depositional event with recent (2(1(0)Pb material, probably related to strong shifts in precipitation and drought cycles in that part of the world. These three examples show that (2(1(0)Pb profiles provide valuable information not only on geochronology, but also related to natural and anthropogenic short term processes, as shown here, but these are not always reported and well understood.
Two state-of-the-art techniques were used to assess the impact of organic loading from fish farming in two fjords of Southern Chile, Pillan and Reñihue Fjords. A sediment profile imaging (SPI) camera ...was deployed and sediment microprofiles (oxygen, H
2S, redox and pH) were measured in undisturbed sediment cores collected using a HAPS corer. Four out of seven stations in Pillan Fjord were found to be severely disturbed: SPI images showed azoic conditions (no apparent Redox Potential Discontinuity layer, no evidence of aerobic life form, presence of an uneaten fish food layer, negative OSI scores). These findings were corroborated by very high oxygen consumption rates (700–1200
mmol m
−
2
day
−
1
), H
2S concentrations increasing quickly within the sediment column and redox potential decreasing towards negative values within a few mm down core. Results for Reñihue Fjord were not so straightforward. SPI images indicated that most of the stations (R3 to R7) presented well-mixed conditions (high apparent RPD layers, presence of infauna, burrows, etc.), but oxygen profiles yielded consumption rates of 230 to 490
mmol m
−
2
day
−
1
and organic carbon mineralization of 2.16 to 4.53
g C m
−
2
day
−
1
. These latter values were close to the limit of aerobic degradation of organic matter although no visible changes were recorded within the sediment column. In view of our findings, the importance of integrating multidisciplinary methodologies in impact assessment studies was discussed.
Global fallout is the main source of anthropogenic radionuclides in the Mediterranean Sea. This work presents
137
Cs,
239+240
Pu and
241
Am concentrations in the water column in the southwest Alboran ...Sea, which was sampled in December 1999. A sediment core was taken at 800 m depth in the area (35°47′ N, 04°48′ W).
210
Pb,
226
Ra,
137
Cs and
239+240
Pu specific activities were measured at multiple depths in the core for dating purposes.
137
Cs and
239+240
Pu profiles did not show defined peaks that could be used as time markers, and they extended up to depths for which the
210
Pb-based constant rate of supply (CRS) dating model provided inconsistent dates. These profiles can be useful to test dating models, understood as particular solutions of a general advection–diffusion problem, if the time series of radionuclide inputs into the sediment is provided. Thus, historical records of depth-averaged
137
Cs and
239+240
Pu concentrations in water, and their corresponding fluxes into the sediment, were reconstructed. A simple water-column model was used for this purpose, involving atmospheric fallout, measured distribution coefficient (
k
d
) values, and a first-estimate of sedimentation rates. A dating model of constant mixing with constant sedimentation rate was applied successfully to three independent records (unsupported
210
Pb,
137
Cs and
239+240
Pu), and provided the objective determination of mixing parameters and mass sedimentation rate. These results provide some insight into the fate of atmospheric inputs to this marine environment and, particularly, into the contribution from the Chernobyl accident.
Dissolved and particulate samples were collected to study the distribution of thorium isotopes (
234Th,
232Th and
230Th) in the water column of the Indian sector of the Southern Ocean (from 42°S to ...47°S and from 60°E to 66°E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess
230Th (
230Th
xs) increases linearly with depth in surface water (0–100 m) and a model was applied to estimate a residence time relative to the thorium scavenging (
τ
scav). Low
τ
scav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the
230Th
xs scavenging efficiency and explain this difference. An innovative coupling between
234Th and
230Th
xs was then used to simultaneously constrain the settling velocities of small (0.6–60 μm) and large (above 60 μm) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60
±
2 m d
−
1
, station 8) than south of the Agulhas Return Current (119
±
23 and 130
±
5 m d
−
1
at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the
230Th
xs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the
230Th
xs concentrations. A specific
230Th
xs transport model is applied in the deep water column and allows us to assess a “travel time” of NADW ranging from 2 to 15 years.
The
210Pb dating method was first introduced by
Goldberg (1963), and since then has been applied to study sediment from lakes, estuaries and coastal marine environments. Hundreds of studies around ...the world have used
210Pb as a geochronological tool in aquatic ecosystems. However little attention has been paid to the potential of this naturally occurring isotope as an environmental tracer of ecological events. Here we report three instances in which
210Pb profiles measured on undisturbed sediment cores from lakes, rivers and fjords show us the potential of
210Pb profile as a tracer of natural and anthropogenic processes. The methodology used here is a suite of techniques combining biogeochemistry (micro-electrodes), paleomagnetism (susceptibility), sediment characteristics (LOI) and visualization (SPI and X-ray) applied to the interpretation of
210Pb profiles. We measured
210Pb profiles on sediments from a river, Cruces River (Chile), which recorded a clear shift in the water chemistry caused by a pulp mill effluent to the river. Here metal mobilization and remobilization of the tracer may be the cause of the observed profile. We also measured
210Pb profiles in sediment from two fjords of Southern Chile (Pillan and Reñihue), the sudden deposition change of fresh
210Pb with depth observed could very well be the result of bioturbation but it occurred in a seafloor area deprived of bioturbators. In this case,
210Pb recorded the onset of aquaculture activities (fish farming) that took place two decades ago. Finally,
210Pb profiles measured in two lakes in the “
pampa Argentina”: Epecuen and Venado showed a particular shape with depth. These profiles apparently registered a sudden depositional event with recent
210Pb material, probably related to strong shifts in precipitation and drought cycles in that part of the world. These three examples show that
210Pb profiles provide valuable information not only on geochronology, but also related to natural and anthropogenic short term processes, as shown here, but these are not always reported and well understood.
The Irish Sea has important fishing grounds shared by several European countries and the quality of its marine environment is being closely monitored. Radioactivity has been of interest particularly ...in relation to authorized discharges from the Sellafield nuclear reprocessing plant on the Cumbrian coast. A "sentinel"-type device, equipped with a NaI (Tl)-based underwater gamma-spectrometer mounted on a stationary buoy, transmitting data through satellite connection, was deployed in the north-western Irish Sea.The data recorded and received in real-time indicated that no detectable significant changes occurred in the concentrations of 137Cs at the mooring site in the period September to November, 2000. Other anthropogenic gamma emitters were not detected.
High-resolution seismic profiles along with physical and sedimentological properties of sediment cores from the Saguenay (Eastern Canada) and Reloncavi (Chile) Fjords allowed the identification of ...several decimeter to meter-thick turbidites. In both fjords, the turbidites were associated with large magnitude historic and pre-historic earthquakes including the 1663
AD (M
>
7) earthquake in the Saguenay Fjord, and the 1960 (M 9.5), 1837 (M
~
8) and 1575
AD major Chilean subduction earthquakes in the Reloncavi Fjord. In addition, a sand layer with exoscopic characteristics typical of a tsunami deposit was observed immediately above the turbidite associated with the 1575
AD earthquake in the Reloncavi Fjord and supports both the chronology and the large magnitude of that historic earthquake. In the Saguenay Fjord, the earthquake-triggered turbidites are sometimes underlying a hyperpycnite associated with the rapid breaching and draining of a natural dam formed by earthquake-triggered landslides. Similar hyperpycnal floods were also recorded in historical and continental geological archives for the 1960 and 1575
AD Chilean subduction earthquakes, highlighting the risk of such flood events several weeks or months after main earthquake. In both fjords, as well as in other recently recognized earthquake-triggered turbidites, the decimeter-to meter-thick normally-graded turbidites are characterized by a homogeneous, but slightly fining upward tail. Finally, this paper also emphasizes the sensitivity of fjords to record historic and pre-historic seismicity.
Display omitted
► Seismic profiles and cores from the Saguenay and Reloncavi Fjords are presented. ► Decimeter to meter-thick turbidites were related to large magnitude earthquakes. ► The normally-graded turbidites are characterized by a homogenous fining upward tail. ► A sand layer interpreted as a tsunami deposit was observed in Reloncavi Fjord. ► Fjords are pristine sites to record historical and pre-historical earthquakes.
A coastal-lake wetland is a type of inland water ecosystem that does not occur very frequently in Chile, of which lakes Budi, Huillinco and Cucao are examples. Textural differences were found in the ...various constituent parts of the coastal lake sediments. Gravel was the smallest fraction and showed significant differences among the three coastal lakes (P < 0.05). Significant differences (P < 0.05) were also recorded for the sand and mud fractions. Lakes Cucao and Huillinco are described as muddy sand, in contrast to Lake Budi, which is described as sandy mud, composed primarily of mud with a smaller percentage of sand. There were statistical differences among the lakes with regard to the contributions of organic matter in each fraction. The macrofauna in these bodies of water belongs to the fauna inhabiting estuary environments in southern Chile. Lake Budi had seven taxa, and lakes Huillinco and Cucao ten species of macrofauna each. There were no significant differences in the number of species captured at the three coastal lakes (P > 0.05), but differences did emerge in comparisons made among the sampling stations. Total abundances showed significant differences (P < 0.05) in comparisons made both among the lakes and the stations. Salinity and grain size could be limiting the density and number of species at the lakes.
The water column deficiencies of 234Th were used to estimate the Particulate Organic Carbon (POC) fluxes in the Indian Sector of the Southern Ocean. Samples were collected in January–February 1999 in ...a frontal zone from 42°S to 47°S and from 60°E to 66°E during the ANTARES 4 cruise. Beta counting was used to measure the 234Th activities onboard. The 234Th export fluxes were estimated from the 234Th/238U disequilibria using a steady state 234Th model. A non-steady state model gave results close to the steady-state model in the Subtropical Zone and could not be used in the Subantarctic Zone due to a strong vertical mixing event. Small and large particles analysis indicated that the POC/234Th ratios decreased when the particle size increased. From the POC/234Th ratios on the large filtered particles, it appears that the POC export fluxes exported below 100m were very low (from 0.10 to 2.53mmolCm−2d−1) compared to those observed in the Southern Ocean and (Deep Sea Res. II 48 (2001) 4275; Deep Sea Res. II 47 (15–16) (2000) 3451; Deep Sea Res. II 44 (1997) 457) and with a strong zonal variation . It is hypothesized that the low POC export fluxes were related to the low predominance of diatoms, characteristic at the end of a bloom period. In this way, the very low POC export observed in the Subtropical Zone suggests an efficient remineralization process and/or a high bacterial activity. Otherwise, a decoupling between the primary production and the POC export derived from 234Th could also explain the low POC export.
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