Florin Filip

The growth of Chilia deltaic lobes reflects a drastic reorganization of the Danube delta that accompanied
its rapid expansion in the late Holocene. Using new cores collected at the apices of the two older Chilia
lobes, together with historical maps and satellite photos, we find that a partial avulsion since 1500
years BP led to a gradual rejuvenation of the Chilia distributary. This process led to the successive infilling
of a lake and a lagoon and subsequently to the construction of an open coast lobe at the Black Sea coast.
The Chilia branch became the largest Danube distributary, reaching its maximum sediment load in the
last 300 years as the southernmost St. George branch lost its previous dominance. Here, we propose that
the intensive deforestation of Danube’s lower watershed leading to this delta reorganization has
historical cultural causes: an increase in sheep and timber demand associated to the Ottoman Empire
expansion in Eastern Europe followed by the adoption of maize agriculture as a result of the Columbian
Exchange. Rapid industrialization-driven damming during the Communist Era led to the current
generalized sediment deficit for the Danube. Under these conditions, the modern Chilia lobe is rapidly
remodeled by waves and may join the Sulina coast to impede navigation on the Sulina canal.

Deltas are particularly vulnerable coasts, affected by changes in both continental and coastal ocean processes. The currently accelerated loss of deltaic lands across the world is primarily due to fluvial sediment starvation following the pandemic construction of river dams and water diversions. However, the influence on deltas of human- or even climate-modulated changes in fluvial sediment discharge has been studied less comprehensively than other controls e.g., the sea-level rise. We examined the Indus delta to understand how the sediment source (i.e., available fluvial sediment) has affected the development of the sediment sink (i.e., the subaerial and subaqueous delta evolution). With an elevated topography and generally arid climate in the watershed, the Indus has been, in natural conditions (i.e., before dams reduced its flow and sediment discharge by over 80%), one of the most important sediment-producing rivers in the world. Bathymetric data show that the Indus shelf morphology exhibits a compound clinoform morphology. Whereas the inner shoreface-connected clinoform has clearly developed as the subaqueous part of the modern Indus delta, the offshore clinoform is either a relict or a contemporaneous prodelta clinoform. Following the reduction in sediment discharge after the late 1950's, the deltaic shoreline in the central part of the delta coast started to recede providing sediment for the southeastern and northwestern coast sectors that remained largely progradational. This differential behavior of the delta shoreline indicates that even a drastic loss of fluvial sediment is initially buffered by an erosional smoothing of the delta coast. New data from onland drilling shows that unlike most Holocene deltas, the Indus delta prograded through the later part of the deglacial sea level rise, starting no later than 12,000 cal. years BP. Neodymium isotope data indicate that sediments comprising the entire Holocene delta originated predominantly in the monsoon-affected Himalayas. This early inception of the Indus delta was probably controlled by an augmented sediment delivery from the Indus basin occurring between 13000 and 9500 years BP during a period of abrupt increase in the intensity of the summer monsoon. Subsequently, a centuries-long phase of reduced precipitations in the Indus basin resulted in an abrupt decrease of the Indus discharge sometimes between 9000 and 8000 years. During this phase, almost the entire early Indus delta was flooded as marine waters penetrated deep inland, providing a grim analog for the future of currently sediment starved river deltas. Furthermore, it will be discussed that this complex evolution of the Indus delta points toward a fundamental change in paradigm for the dynamics deltas at centennial to millennial time scales and for interpreting the sedimentary architecture of transgressive and highstand deltaic deposits.

The collapse of the Bronze Age Harappan, one of the earliest urban
civilizations, remains an enigma. Urbanism flourished in the western
region of the Indo-Gangetic Plain for approximately 600 y, but
since approximately 3,900 y ago, the total settled area and settlement
sizes declined, many sites were abandoned, and a significant
shift in site numbers and density towards the east is recorded. We
report morphologic and chronologic evidence indicating that fluvial
landscapes in Harappan territory became remarkably stable
during the late Holocene as aridification intensified in the region
after approximately 5,000 BP. Upstream on the alluvial plain, the
large Himalayan rivers in Punjab stopped incising, while downstream,
sedimentation slowed on the distinctive mega-fluvial ridge,
which the Indus built in Sindh. This fluvial quiescence suggests a gradual
decrease in flood intensity that probably stimulated intensive
agriculture initially and encouraged urbanization around 4,500 BP.
However, further decline in monsoon precipitation led to conditions
adverse to both inundation- and rain-based farming. Contrary to
earlier assumptions that a large glacier-fed Himalayan river, identified
by some with the mythical Sarasvati, watered the Harappan
heartland on the interfluve between the Indus and Ganges basins,
we show that only monsoonal-fed rivers were active there during
the Holocene. As the monsoon weakened, monsoonal rivers gradually
dried or became seasonal, affecting habitability along their
courses. Hydroclimatic stress increased the vulnerability of agricultural
production supporting Harappan urbanism, leading to settlement
downsizing, diversification of crops, and a drastic increase in
settlements in the moister monsoon regions of the upper Punjab,
Haryana, and Uttar Pradesh.

Over the last half century, while the total sediment load of the Danube dramatically decreased due to
dam construction on tributaries and its main stem, a grand experiment was inadvertently run in the
Danube delta: the construction of a dense network of canals, which almost tripled the water discharge
toward the interior of the delta plain. We use core-based and chart-based sedimentation rates and
patterns to explore the delta transition from the natural to an anthropogenic regime, to understand the
effects of far-field damming and near-field channelization, and to construct a conceptual model for delta
development as a function sediment partition between the delta plain and the delta coastal fringe. We
show that sediment fluxes increased to the delta plain due to channelization counteracting sea level rise.
In turn, the delta coastal fringe was most impacted by the Danube’s sediment load collapse. Furthermore,
we suggest that morphodynamic feedbacks at the river mouth are crucial in trapping sediment near the
coast and constructing wave-dominated deltas or lobes. Finally, we suggest that increased
channelization that mimics and enhances natural processes may provide a simple solution for keeping
other delta plains above sea level and that abandonment of wave-dominated lobes may be the most long
term efficient solution for protecting the internal fluvial regions of deltas and provide new coastal
growth downcoast.

Over the last century humans have altered the export of fluvial materials leading to significant changes in morphology, chemistry, and biology of the coastal ocean. Here we present sedimentary, paleoenvironmental and paleogenetic evidence to show that the Black Sea, a nearly enclosed marine basin, was affected by land use long before the changes of the Industrial Era. Although watershed hydroclimate was spatially and temporally variable over the last ,3000 years, surface salinity dropped systematically in the Black Sea. Sediment loads delivered by Danube River, the main tributary of the Black Sea, significantly increased as land use intensified in the last two millennia, which led to a rapid expansion of its delta. Lastly, proliferation of diatoms and dinoflagellates over the last five to six centuries, when intensive deforestation occurred in Eastern Europe, points to an anthropogenic pulse of river-borne nutrients that radically transformed the food web structure in the Black Sea. T he development of human civilization is the most significant environmental event of the last 10,000 years 4. The global impact of human activities has come into focus as fossil fuel-based industrialization has accelerated over the last century, but the expansion of agriculture may have affected the entire Earth System as early as 8000 years ago 5. In the ocean, anthropogenic impacts include increasing temperatures, sea level rise, eutrophication and acidification 6. Evidence for long term, pre-industrial anthropogenic modifications of the oceans remains sparse 1,7,8 , although humans became active agents of continental denudation thousands of years ago 9. However, 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 6. Regional seas 10 provide the best opportunity to examine early anthropogenic changes in the marine environment. Their amplified reactivity to continental processes is the result of the limited water exchange with the open ocean and the relatively large fluvial contribution to their overall water budget. We selected the Black Sea to examine if pre-industrial land use was recorded by sedimentation patterns and sedimentary biogeochemistry (Fig. 1A). The landlocked Black Sea is a sensitive recorder of continental signals as demonstrated by drastic and repeated alteration of its ecosystem over the past half century 2,11. Although evaporation exceeds precipitation, the freshwater budget of the Black Sea is positive due to river runoff 12. Low salinity water is exported at the surface toward the Mediterranean through the narrow and shallow Straits of Bosporus; denser water from the Sea of Marmara flows into the Black Sea as an undercurrent. River runoff drives much of the seasonal and interannual variability in salinity and sea level, which in turn affects the water exchange through the Bosporus 13. The Danube River, draining ,30% of central and eastern Europe (Fig. 1A), provides over 60% of the entire runoff reaching the Black Sea 12. Because fluvial nutrients feed the surface mixed layer that accounts for only 13% of the entire basin volume, there is a strong coupling between Danube fluxes and the Black Sea's biogeochemistry and ecology 2,11. A vigorous cyclonic mean surface circulation with superimposed mesoscale eddies 13,14 redistributes nutrients in the surface mixed layer at multiannual time scales 13 , whereas nutrient exchanges with deeper suboxic and anoxic layers are secondary in importance 13. After the 1960s, the Black Sea was transformed within a decade into the largest eutrophic water body in the world, by discharge of industrial, domestic and agricultural waste coming primarily from the Danube watershed 11. In addition, construction of dams within the Danube watershed substantially reduced dissolved silicate

The collapse of the Bronze Age Harappan, one of the earliest urban
civilizations, remains an enigma. Urbanism flourished in the western
region of the Indo-Gangetic Plain for approximately 600 y, but
since approximately 3,900 y ago, the total settled area and settlement
sizes declined, many sites were abandoned, and a significant
shift in site numbers and density towards the east is recorded. We
report morphologic and chronologic evidence indicating that fluvial
landscapes in Harappan territory became remarkably stable
during the late Holocene as aridification intensified in the region
after approximately 5,000 BP. Upstream on the alluvial plain, the
large Himalayan rivers in Punjab stopped incising, while downstream,
sedimentation slowed on the distinctive mega-fluvial ridge,
which the Indus built in Sindh. This fluvial quiescence suggests a gradual
decrease in flood intensity that probably stimulated intensive
agriculture initially and encouraged urbanization around 4,500 BP.
However, further decline in monsoon precipitation led to conditions
adverse to both inundation- and rain-based farming. Contrary to
earlier assumptions that a large glacier-fed Himalayan river, identified
by some with the mythical Sarasvati, watered the Harappan
heartland on the interfluve between the Indus and Ganges basins,
we show that only monsoonal-fed rivers were active there during
the Holocene. As the monsoon weakened, monsoonal rivers gradually
dried or became seasonal, affecting habitability along their
courses. Hydroclimatic stress increased the vulnerability of agricultural
production supporting Harappan urbanism, leading to settlement
downsizing, diversification of crops, and a drastic increase in
settlements in the moister monsoon regions of the upper Punjab,
Haryana, and Uttar Pradesh.

Accelerated sea-level rise and anthropogenic stresses on large delta-forming rivers have led to intense pressure on deltas around the world. To address issues of maintenance and restoration natural deltaic processes need to be understood in detail. The evolution of Danube delta was studied by numerous researchers in the last century and a vast majority of these studies were driven by development concerns for navigation, resource exploration, agriculture, and fisheries. In an ongoing study on paleoenvironmental aspects of the Danube delta, we collected a series of sediment cores (1 to 6 m long) along the Chilia distributary. Sedimentological, paleontological, and geochemical analyses combined with AMS radiocarbon on in situ articulated mollusks lead us to suggest that the delta lobes that the Chilia arm built are much younger than previously thought. Furthermore, we can now ascertain that Chilia I lobe grew as a lacustrine delta, whereas Chilia II developed as a bayhead delta with a lagoon. In contrast, the modern Chilia III lobe has been building in the open Black Sea since the 18th Century.