Oceanography Quick-look: the Black Sea
This is the first in a series of short oceanographic studies using our Ocean API. To begin, we explore the Black Sea — a large inland sea bordered by Eastern Europe and Western Asia, with coastlines along Ukraine, Turkey, Bulgaria, Romania, Russia, and Georgia. It connects to the Mediterranean Sea via the Bosporus Strait, the Sea of Marmara, and the Dardanelles Strait.
There are fascinating hypotheses on how the Black Sea was formed. The “Black Sea deluge hypothesis” suggests that it was formed or significantly altered by a catastrophic flooding event around 7,600 years ago whereby the Mediterranean Sea broke through the Bosporus Strait, leading to a massive influx of saltwater into what was then a much smaller, freshwater lake or inland sea.
Proponents of the hypothesis argue that this sudden flooding could have submerged vast areas of land, displacing ancient populations and possibly been passed down through generations and eventually forming the basis for the story of Noah’s Ark in the Bible.
In addition to having biblical proportions, the Black Sea exhibits a unique hydrological system:
Freshwater from rivers like the Danube, Dniester, and Dnieper flows into the Black Sea, creating a layer of less dense, less saline water at the surface. An outflow of this fresher Black Sea surface water takes place near the surface of the basin into the Bosphorus and Dardanelles Strait.
Simultaneously, an inflow of more saline water from the Aegean Sea through the same straits occurs at depth.
There is very low and slow mixing between these strata (with cycling times in the order of thousands of years!) resulting in the lower depths becoming among the most anoxic (sans oxygen) in the planet. By contrast, interaction between the atmosphere and the upper layer results in an oxygen rich surface layer that supports a vibrant ecosystem that is fed by large swirls of phytoplankton blooms and sports Black Sea bottlenose dolphin populations.
Now let’s explore oceanographic data from the Black Sea using the Amentum Scientific Ocean API. Firstly, we create salinity maps with 0.1 degree resolution at different depths using the nemo/phys API endpoint which, in turn, provides programmatic access to data from the Copernicus Marine Service. The NEMO (Nucleus for European Modelling of the Ocean) is a general model of ocean circulation that is composed of several models. Our API endpoint provides access to the Operational Mercator-Ocean biogeochemical and physical global ocean analysis and forecast system. The following analysis is based on historical model predictions for 21 August 2024.
The figure above shows the spatial distribution of salinity at the surface of the Black Sea, ranging from 5 to 20 PSU. By comparison, the average salinity of the ocean is approximately 35 PSU, so the Black Sea has very low salinity— a result of the aforementioned freshwater sources feeding into it. Note the very low salinity near the rivers, particularly around the Sea of Azov in the upper right of the map.
The salinity is higher at a depth of 200m (above) and continues to increase at a depth of 400 m (below). The lack of data in some areas is due to the bathymetry of the Black Sea (shallower than 200m in those areas).
So just how deep is the Black Sea? We can easily plot that depth information using the gebco endpoint of the Ocean API. GEBCO is the General Bathymetric Chart of the Oceans which aims to be the most authoritative publicly available baythmetry datasets of the world’s oceans, releasing a new grid each year. The Black Sea bathymetry map is shown below.
That is a deep blue sea indeed (note the log scale)! This showcases another interesting feature of the Black Sea: the Euxine abyssal plain. It is a flat, extensive plain located in the central part of the Black Sea. Abyssal plains are formed by the deposition of fine sediments over long periods, which cover the underlying oceanic crust and create a smooth, even surface.
The Black Sea’s anoxic waters have preserved ancient organic materials, including shipwrecks and other human artefacts, in a remarkable state on the Euxine plain due to the absence of oxygen-dependent organisms that would typically cause decay. One such example is the wreck of the Sinop D discovered in 2000, with its hull and cargo intact and radiocarbon dated to 410–520 CE.
So where is the deepest point? According to the bathymetric dataset, the sea is 2.235 km at its deepest point, which is located at latitude/longitude of 42.47,33.86 deg — south of Ukraine’s Crimean peninsula (‘X’ marks the spot in the above figure).
Salinity and temperature profiles are shown below (left and centre, respectively) and both drop off rapidly with depth, as expected.
The right-most profile above shows a very important biogeochemical property, Chlorophyll-a, which is the pigment responsible for photosynthesis in phytoplankton. By measuring the concentration of Chl-a, scientists can estimate the amount of phytoplankton in a given area. Since phytoplankton are the primary producers in the ocean, generating organic material from sunlight, Chl-a levels are directly linked to the overall productivity of the marine ecosystem. We see fine structure in the above Chl-a depth profile, with peak concentration at depth (the so called Deep Chlorophyll Maximum). Why the fine structure? Because phytoplankton populations are driven by the availability of light and nutrients, and these vary spatially. They also vary temporally, having diurnal variation (time of day) and seasonal variation. The figure below zooms in on that fine structure.
Ricour et al performed an in-depth (pun intended) scientific study that classified the type and seasonal variation of deep chlorophyll maximum (DCM) profiles at various locations in the Black Sea using bio-Argo floats, rather than the model-based data considered here. The DCM above is consistent with the sigmoid + gaussian analytical form from Figure 1 of their publication, being the dominant type for August (see their Figure 3).
And that’s where we’ll drop anchor. The Python code used to create the above images is available here. It leverages a couple of packages we created to make it easier to use our APIs: the first abstracts complexity of making asynchronous web API calls in Python; the second does the same for plotting geospatial maps. Reproducing the plots requires subscriptions to our APIs. Sign up here for a 14 day free trial and an API key needed for access.
Looking forward to sharing more insights in the next oceanography quick-look.
Go well,
⚓ Amentum Scientific 🌊
