The Danube's watershed is delimited by a continuous white line, its
course is highlighted in blue, and the Danube delta is shown in green.
The Danube drains most of the Central and Eastern Europe to dominate the
freshwater and sediment budget of the Black Sea. A sediment core shown
by a white circle was used to reconstruct the salinity and ecosystem
composition in the Black Sea over the last 7000 years. Credit: Liviu Giosan, Woods Hole Oceanographic Institution; Stefan Constantinescu, University of Bucharest
When WHOI geologist Liviu Giosan first reconstructed the history of
how the Danube River built its delta, he was presented with a puzzle.
In the delta's early stages of development, the river deposited its
sediment within a protected bay. As the delta expanded onto the Black
Sea shelf in the late Holocene and was exposed to greater waves and
currents, rather than seeing the decline in sediment storage that he
expected, Giosan found the opposite. The delta continued to grow. In
fact, it has tripled its storage rate.
If an increase in river runoff was responsible for the unusual rapid
build up of sediment in the delta, says Giosan, the question is, "Was
this extraordinary event in the Danube delta felt in the entire Black
Sea basin? And if so, what caused it?"
In answering those questions, Giosan and an international team of
collaborators including environmental engineers, modelers,
paleogeographers, and paleobiologists pieced together a unique history
of the region that ultimately provides evidence for a transformative
impact of humans on the Black Sea over hundreds, if not thousands of
years. The study was published on August 30 in Scientific Reports, a new online journal of the Nature Publishing Group.
The largest and longest river in the European Union, the Danube is
the source of over 60 percent of the freshwater running into the Black
Sea, and therefore is a dominant factor in the biogeochemistry of this
basin. Because the Black Sea is nearly enclosed, changes in its
hinterland provoked by climate or people should be readily reflected
there.
The research team reconstructed a 9000-year record of the delta's
growth, and through various scientific techniques, developed and mapped
against it a record of changes in the Black Sea's salinity, nutrients,
and relative abundance of its ecosystem's major phytoplankton groups.
Finally, they also examined the history of land use in the greater
Danube watershed.
To reconstruct the salinity, the WHOI team analyzed sediments
containing highly resistant organic compounds called alkenones, which
are uniquely produced by Emiliania huxleyi – the same
photosynthetic organism oceanographers study to determine past sea
surface temperatures. By examining the ratio of two hydrogen isotopes in
the alkenones, they were able to map the salinity trend in the Black
Sea over the last 6,500 years.
A visual inspection of the core clearly shows different types of
sediment deposited over time. Analysis of the first section (far left),
determined the sediment was deposited over the past 2515 years and
contained E huxleyi coccoliths. The second core section sediment
was deposited 2515-7550 years ago. The last three sections (middle to
right) contain calcite-rich, lacustrine or lake-related, sediments
dating back 7550-~13,000 years. Credit: Dimitri Dimitrov, Institute of Oceanology, Bulgarian Academy of Sciences (IO-BAS)
"One of the isotopes, deuterium, is not very common in nature,"
explains WHOI marine paleoecologist Marco Coolen, "and it doesn't
evaporate as easily as other isotopes. Higher ratios of deuterium are
indicative of higher salinity."
Salinity began to rise in the Black Sea about 9000 years ago, when
the ocean invaded the previously freshwater lake through the Straits of
Bosporus, and continued to increase until approximately 3000 years ago,
when the levels approached normal ocean values.
"But the trend since then is counter-intuitive," says Giosan. "The
entire basin freshened, especially in the last 1,500 years." The likely
explanation points to a increase in the river input combined with a
reduced evaporation as the climate cooled at the time, he adds.
Such an influx of nutrient-rich river water would affect the
composition of the ecosystem, and would be reflected by the ecological
history of major phytoplankton groups in the Black Sea, such as diatoms
and dinoflagellates.
Researchers traditionally conduct paleo studies of phytoplankton by
using a microscope to count the fossil skeletons found in sediment
cores. But, this method is limited because some phytoplankton leave no
fossils, so, instead, Coolen looked for sedimentary genetic remains of
the past inhabitants of the Black Sea water column.
"DNA offers the best opportunity to learn the past ecology of the
Black Sea," says Coolen. "Calcareous and organic-walled dinocysts are
frequently used to reconstruct past environmental conditions, but 90
percent of the dinoflagellate species do not produce such diagnostic
resting stages, but their DNA remains in the fossil record."
His analysis found that major phytoplankton groups in the Black Sea
changed drastically over the last millennium. Beginning about 500 years
ago, a rise in diatoms suggests there was more silicate available in the
Black Sea. The team at WHOI also found an increase in the abundance of
dinoflagellate DNA beginning about 650 years ago. Both of these changes
are consistent with increased nutrient availability that would occur as a
result of human activity – the clearing of forests and increased
agriculture. Interestingly, a significant drop in diatom DNA occurred in
recent times when more recent human activity – the damming of the
rivers – entered the scene.
Woods Hole Oceanographic Institution geologist Liviu Giosan (left) Emil
Vespremeanu (in blue) and Ionutz Ovejanu (yellow) of the University of
Bucharest use an auger to bore holes in the Danube Delta to reconstruct
its growth history. Credit: Liviu Giosan
The final piece of the picture comes from a model developed by Jed
Kaplan, a scientist at Ecole Polytechnique Federale de Lausanne in
Switzerland. Kaplan looked at the expansion of lands cleared for
agriculture in the Danube watershed as population grew and farming
technology became more advanced over the last 7000 years ago. His
findings agree very well with recently published studies of the relative
abundance of grass pollen over tree pollen and the appearance of
charcoal in the sediment record from clearing large swaths of trees by
burning. All of these land signals point to human deforestation and
increased agriculture.
The model suggests that deforestation of the lower basin of the
Danube rapidly accelerated over the last millennium and especially in
the last 500 years. In contrast in the western sector of the watershed,
farther from the Black Sea, deforestation was already substantial as the
Roman Empire expanded there.
"This story started by looking at how Danube River built its delta
in the Black Sea," says Giosan. "But together these indicators tell a
story of changing land use that ultimately changed the ecosystem of an
entire sea."
The ability to define baselines and separate natural variability
from human-induced changes is essential for understanding biogeochemical
cycles and managing marine ecosystems, which ultimately depends on the
detection and attribution of long term environmental trends, the authors
write. "Our ancestors changed that baseline a long time ago in the
Black Sea" says Giosan. "And they probably fertilized other coastal seas
with nutrients inadvertently stripped from soils much earlier than we
think."
Meanwhile, the Danube delta remains an extraordinarily lush, vibrant
habitat, home to hundreds of species of birds and fish. "This is a
unique piece of land – the Danube delta – blessed, productive, and
important for countless forms of life," adds Giosan. "What is amazing to
me is that we owe much of this biodiversity to our own history of
changing Nature."
The Woods Hole Oceanographic Institution is a private, non-profit
organization on Cape Cod, Mass., dedicated to marine research,
engineering, and higher education. Established in 1930 on a
recommendation from the National Academy of Sciences, its primary
mission is to understand the oceans and their interaction with the Earth
as a whole, and to communicate a basic understanding of the oceans'
role in the changing global environment.
Contact: WHOI Media Relations
media@whoi.edu
508-289-3340
Woods Hole Oceanographic Institution
