Edwige Pons-Branchu

Gypsum caves as indicators of climate-driven river incision and aggradation in a rapidly uplifting region

Detailed geomorphological analysis has revealed that subhorizontal gypsum caves in the Northern Apennines (Italy) cut across bedding planes. These cave levels formed during cold periods with stable river beds, and are coeval with fluvial terraces of rivers that flow perpendicular to the strike of bedding in gypsum monoclines. When rivers entrench, renewed cave formation occurs very rapidly, resulting in the formation of a lower level. River aggradation causes cave alluviation and upward dissolution (paragenesis) in passages nearest to the river beds. The U-Th dating of calcite speleothems provides a minimum age for the formation of the cave passage in which they grew, which in turn provides age control on cave levels. The ages of all speleothems coincide with warmer and wetter periods when CO 2 availability in the soils covering these gypsum areas was greater. This climate-driven speleogenetic model of epigenic gypsum caves in moderately to rapidly uplifting areas in temperate regions might be generally applicable to karst systems in different geological and climatic conditions.

South Atlantic intermediate water advances into the North‐east Atlantic with reduced Atlantic meridional overturning circulation during the last glacial period

The Nd isotopic composition (eNd) of seawater and cold-water coral (CWC) samples from the Gulf of Cadiz and the Alboran Sea, at a depth of 280–827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of Cadiz over the past 40 ka. eNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cadiz reveals relatively constant values (−8.6 to −9.0 and −9.5 to −10.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; eNd ∼ −9.4) and Mediterranean Sea Water (MSW; eNd ∼ −9.9). In contrast, glacial eNd values for CWC collected at thermocline depth (550–827 m) in the southern Gulf of Cadiz display a higher average value (−8.9u2009±u20090.4) compared to the present-day value (−11.7u2009±u20090.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic eNd values (∼ −8) at 19, 23–24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW eNd values remained stable during the last glacial period, it is suggested that these radiogenic eNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.

Urbanization impact on sulfur content of groundwater revealed by the study of urban speleothem-like deposits: Case study in Paris, France

Speleothem-like deposits that develop underground in urban areas are an archive of the environmental impact of anthropic activities that has been little studied so far. In this paper, the sulfate content in shallow groundwater from northern Paris (France) is compared with the sulfur content in two 300-year-old urban carbonate deposits that grew in a historical underground aqueduct. The present-day waters of the aqueduct have very high sulfur and calcium contents, suggesting pollution from gypsum dissolution. However, geological gypsum levels are located below the water table. Sulfur content was measured by micro-X-ray fluorescence in these very S-rich carbonate deposits (0.5 to 1% of S). A twofold S increase during the second half of the 1800s was found in both samples. These dates correspond to two major periods of urbanization above the site. We discus three possible S sources: anthropic sources (industries, fertilizers…), volcanic eruptions and input within the water through gypsum brought for urbanization above the studied site (backfill with quarry waste) since the middle of the 19th century. For the younger second half of the studied section, S input from gypsum brought during urbanization was confirmed by the study of isotopic sulfur composition (δ34S=+15.2‰ at the top). For the oldest part, several sulfur peaks could be related to early industrial activity in Paris, that caused high local air pollution, as reported in historical archives but also to historical gypsum extraction. This study provides information on the origin and timing of the very high SO42- levels measured nowadays within the shallow groundwater, thus demonstrating the interest in using carbonate deposits in urban areas as a proxy for the history of urbanization or human activities and their impact on water bodies.

Surface ocean pH variations since 1689 CE and recent ocean acidification in the tropical South Pacific

Increasing atmospheric CO2 from man-made climate change is reducing surface ocean pH. Due to limited instrumental measurements and historical pH records in the world’s oceans, seawater pH variability at the decadal and centennial scale remains largely unknown and requires documentation. Here we present evidence of striking secular trends of decreasing pH since the late nineteenth century with pronounced interannual to decadal–interdecadal pH variability in the South Pacific Ocean from 1689 to 2011 CE. High-amplitude oceanic pH changes, likely related to atmospheric CO2 uptake and seawater dissolved inorganic carbon fluctuations, reveal a coupled relationship to sea surface temperature variations and highlight the marked influence of El Niño/Southern Oscillation and Interdecadal Pacific Oscillation. We suggest changing surface winds strength and zonal advection processes as the main drivers responsible for regional pH variability up to 1881 CE, followed by the prominent role of anthropogenic CO2 in accelerating the process of ocean acidification.Ocean acidification due to the industrial era is a major marine environmental concern, yet little is known on the historical ocean pH changes prior to human influence. Here, Wu et al. show that tropical South Pacific seawater pH is linked to ENSO pacing and has recently been decreasing rapidly.

Imprint of Holocene Climate Variability on Cold‐Water Coral Reef Growth at the SW Rockall Trough Margin, NE Atlantic

U-Th ages and temperatures derived from Li/Mg have been measured on coral fragments of Lophelia pertusa and Madrepora oculata collected from two sediment cores, which were taken from cold-water coral (CWC) mounds at 700-790 m water depth at the SW Rockall Trough margin. Our data, combined with previous published data, have allowed us to first estimate the occurrence of CWC at the SW Rockall Trough margin during the Holocene and, second, to better constrain the environmental conditions driving variability in CWC growth. CWC abundance is marked by a pronounced increase in the mid-Holocene (similar to 6 ka) and is modulated by millennial-scale variability throughout the late-Holocene. The mid-Holocene proliferation of CWC coincides with lowest IRD abundances and a major reorganization of the circulation at thermocline depth in the Rockall Trough, marked by the progressive replacement of the fresh-cold Sub-Arctic Intermediate Water (SAIW) by the saltier and nutrient-rich Eastern North Atlantic Water (ENAW). This event must have established a modern-like winter mixed layer and thermocline structure, generating suitable conditions for enhanced surface productivity, downslope transport of food particles, bottom current acceleration at mound depth and thus CWC growth. Several short time intervals of decreased CWC occurrences closely match prominent increases in North Atlantic drift ice and storminess in Northern Europe. We, therefore, propose that high detrital supply and/or changes in the vertical density gradient associated with millennial-scale ice-rafted detritus (IRD) events are the likely controlling factors for CWC growth and subsequent mound formation on the SW Rockall Trough margin.

Seismic hazards implications of uplifted Pleistocene coral terraces in the Gulf of Aqaba

The Gulf of Aqaba transform plate boundary is a source of destructive teleseismic earthquakes. Seismicity is concentrated in the central sub-basin and decreases to both the north and south. Although principally a strike-slip plate boundary, the faulted margins of the Gulf display largely dip-slip extensional movement and accompanying footwall uplift. We have constrained rates of this uplift by measurements of elevated Pleistocene coral terraces. In particular the terrace that formed during the last interglacial (~125u2009ka) is found discontinuously along the length of the Gulf at elevations of 3 to 26u2009m. Global sea level was ~7u2009m higher than today at 125u2009ka indicating net maximum tectonic uplift of ~19u2009m with an average rate of ~0.015u2009cm/yr. Uplift has been greatest adjacent to the central sub-basin and like the seismicity decreases to the north and south. We suggest that the present pattern of a seismically active central region linked to more aseismic areas in the north and south has therefore persisted for at least the past 125u2009kyr. Consequently the potential for future destructive earthquakes in the central Gulf is greater than in the sub-basins to the north and south.