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| Titel |
Dense water cascading, bottom currents and sediment wave formation at the exit of the Bari canyon (Southern Adriatic Sea, Italy) |
| VerfasserIn |
Leonardo Langone, Stefano Miserocchi, Alfredo Boldrin, Margherita Turchetto, Federica Foglini, Fabio Trincardi |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042745
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| Zusammenfassung |
| The dense water forming in the North Adriatic (NAdDW) spreading southward
along the Italian continental shelf, sinks in the Southern Adriatic basin through
particular cascading events. Such events are seasonal, occurring specially in April,
with variable intensity. These phenomena control the water mass mixing, the deep
ocean ventilation, the behaviour of deep ecosystems, the formation of complex
erosive and depositional bedforms and the abyssal export and burial of nutrients and
carbon.
Because of the NadDW formation is linked to climate factors (frequency, duration and
size of Bura winds), the temporal variations of the NadDW dispersion into the Southern
Adriatic allow to make inferences of the impact of recent climate changes on the ecosystems
of the deep Mediterranean Sea.
Previous research projects (EuroStrataform, HERMES) acquired a large data
set of bathymetric, side-scan sonar (TOBI) and Chirp sonar profiles, which were
used to build detailed morpho-bathymetric maps of the Southern Adriatic margin.
There, the seabed is extremely complex, characterized by a large variety of bedforms
(sediment waves, erosive scours, longitudinal furrows and giant comet marks).
A branch of the cascading NAdDW is confined and accelerated through the Bari
canyon where it produces a strong current capable of reaching down-slope velocities
greater than 60 cm s-1 near the bottom at ~600 m of water depth, eroding the
canyon thalweg and entraining large amounts of fine-grained sediment. At the exit of
the canyon, in water depth greater than 800 m, the current becomes less confined,
spreads laterally and generates an 80-km2-wide field of mud waves; these bedforms
migrate up current and show amplitudes up to 50 m and wavelengths of about 1
km.
Cruise IMPACT-09 of RV Urania was carried out in the Southern Adriatic Sea from
17-30 March 2009 with main scope of studying the impact of NadDW cascading
events on the deep ecosystems of the Southern Adriatic. Experiments planned in the
cruise IMPACT-09 followed multiple directions: 1) stations along a bathymetric
transect and sites studied in previous projects (HERMES, VECTOR, BIOFUN) were
re-occupied in order to identify changes of biodiversity and ecosystem functioning; 2) a
moored array instrumented with an automatic sediment trap, two current meters, a
turbidity sensor and a conductivity and temperature recorder was deployed on the
sediment wave field in order to monitor the seasonal variations of food supply and
hydrodynamics. Furthermore, a short deployment of the mooring was carried out
between 15-24 March, 2009 in order to optimize the position of the instrumented
array.
Here, we will show the first 6-month results of the mooring experiment, which
is planned to last longer than two years. Aims are to verify is the consistence of
present-day currents (speed and direction) with respect to the bedforms located at
the exit of the canyon. Alternatively, these structures must be considered relict,
evidencing past high-energy periods. Unfortunately (at least for this research), winter
2009 was mild and particularly wet. The Po river discharge remained relatively
high through all the winter and peaked in late April-early May. Hence, the quantity
and the characteristics of the produced NadDW made not likely the occurrence of
dense water cascading in the Southern Adriatic. In fact, near-bottom currents at
the mooring site were low, never exceeding 20 cm s-1. Currents were generally
quasi-parallel to the bedform orientation (~135Ë N). Nevertheless, during peak
periods, the current direction slightly shifted northward, becoming more oblique with
respect to the sediment wave crests, the water turbidity showed small amplitude
peaks, and the water temperatures recorded the lowest values of the time-series. |
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