Overestimation of short-term coastal cliff retreat rates in the eastern Mediterranean resolved with a sediment budget approach

Our understanding of sea-cliff erosion processes and their response to recent and/or projected environmental changes such as sea-level rise, climate change and anthropogenic development hinges on our ability to quantify sea-cliff retreat rates and their variability through time. Here, we focus on Israel's Mediterranean ‘Sharon’ sea-cliff as a case study for examining the significance of recent short-term (i.e. annual to decadal) cliff-top retreat rates that appear to exceed longer-term rates of ‘background’ (i.e. centennial to millennial) retreat by 1–2 orders of magnitude. We demonstrate that an inherent sampling bias in rate estimates inferred from observation intervals shorter than process episodicity can also explain such a pattern. This potential ambiguity leads to a striking paradox where despite highly accurate and robust documentation of recent cliff-top retreat, such as that obtained from aerial photographs and/or instrumental surveys, the short-term retreat rates of episodically retreating sea cliffs remain poorly constrained. To address this key data gap along the Sharon sea cliff we employed a sediment budget approach that focuses on quantifying the continuous wave scouring of cliff-collapsed material from the shore platform as a rate-limiting process for episodic retreat of the cliff above. We used four high-resolution (0.5 m/pixel) airborne LiDAR data sets acquired between 2006 and 2015 to determine short-term maximum retreat rates of up to ~0.08 m/yr during this nine-year period. These modern retreat rates compare to the cliff's background retreat rate of 0.03 to 0.09 m/yr since the mid-Holocene, as determined herein from multiple geologic and archeological observations. Our results demonstrate that previously reported twentieth century cliff-top retreat rates for this sea cliff, which range up to values of several meters per year, are biased and that sea-cliff erosion rates have not yet been significantly impacted by recent environmental changes in the eastern Mediterranean basin, such as the restriction of sediment supply following emplacement of the Nile's Aswan dam system. © 2018 John Wiley & Sons, Ltd.     

Spatial and temporal variability in nutrient concentrations in Liverpool Bay,a temperate latitude region of freshwater influence

This paper presents data for the temporal and spatial distribution of nutrients in Liverpool Bay between 2003 and 2009 and an analysis of inputs of nutrients from the major rivers. The spatial distribution of winter nutrient concentrations are controlled by the region of freshwater influence (ROFI) in Liverpool Bay through the mixing of riverine freshwater and Irish Sea water, with strong linear relationships between nutrient concentration and salinity between December and February. The location of highest spring and summer phytoplankton biomass reflects the nutrient distributions as controlled by the ROFI. Analysis of 7 years of data showed that the seasonal cycle of winter maximum nutrient concentrations in February and drawdown in April/May is a recurrent feature of this location, with the timing of the drawdown varying by several weeks between years. A comparison of observed nutrient concentrations in Liverpool Bay with those predicted from inputs from rivers has been presented. Nutrient concentrations in the rivers flowing into Liverpool Bay were highly variable and there was reasonable agreement between predicted freshwater nutrient concentrations using data from this study and riverine nutrient concentrations weighted on the basis of river flow, although the exact nature of mixing between the rivers could not be determined. Predicted Irish Sea nutrient concentrations in the winter were lower than those reported for the input waters of the North Atlantic, supporting findings from previous work that nitrogen is lost through denitrification in the Irish Sea.     

Effects of polychlorinated biphenyls on liver function and sexual characteristics in Japanese medaka (Oryzias latipes)

The effects of polychlorinated biphenyls (PCBs) on liver function and their differences between sexes were analyzed in Japanese medaka (Oryzias latipes) exposed to PCB126 or Kanechlor-400 (KC-400) using microarray. PCB exposure induced vitellogenin 1 expression in female medaka while suppressing choriogenin genes, which suggests that the effects of PCBs on estrogen-responsive genes do not occur directly through an estrogen receptor-mediated pathway. Reduction of androgen receptor alpha expression was also observed, and the gene expression pattern in PCB-exposed males changed to become more similar to that of females. Furthermore, changes in glycolysis-related genes indicate that PCB exposure might enhance glucose production via gluconeogenesis in the liver of medaka. Taken together, our results suggest that PCBs disrupt the endocrine system, especially androgen function, and may have the potential to cause demasculinizing effects. Additionally, induction of gluconeogenesis might be a response to maintain glucose levels consumed as a result of PCB exposures.     

Sand dunes as a major proximal dust source for late Pleistocene loess in the Negev Desert, Israel

Grain size analyses of three hilltop, primary eolian loess sequences in the Negev desert, southern Israel, show a bimodal grain-size distribution at 50-60 μm and 3-8 μm. Using analyses of mineralogy and OSL ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time, suggesting an enhancement of a time-transgressive proximal dust source compared to a distal, Saharan fine-grain dust. The only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into Sinai and the Negev concurrently with the loess accretion during the late Pleistocene as a result of the exposure of the Mediterranean shelf. We therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea. This relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as Northern Africa, China and Australia.     

The role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts

Soils in similar geomorphic settings in hyperarid deserts (< 50 mm yr⁻¹) should have similar characteristics because a negative moisture balance controls their development. However, Reg soils in the hyperarid southern Negev and Namib deserts are distinctly different. Soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics. Annual rainfall amount can be similar (15-30 mm), but storm duration can drastically alter Reg soil properties in deserts. The cooler fall/winter and dry hot summers of the southern Negev Desert with a predominance brief (≤ 1 day) rainstorms result in gypsic-saline soils without any calcic soil horizon. Although the Namib Desert receives only 50-60% of the southern Negev annual rainfall, its rainstorm duration is commonly 2-4 days. This improves leaching of the top soil under even lower annual rainfall amount and results in weeks-long grass cover. The long-term cumulative effect of these rare rain-grass relationships produces a calcic-gypsic-saline soil. The development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil-moisture regime in deserts, and has important implications for the use of key desert soil properties as proxies in paleoclimatology.     

Climate change mitigation: trade-offs between delay and strength of action required

Climate change mitigation via a reduction in the anthropogenic emissions of carbon dioxide (CO₂) is the principle requirement for reducing global warming, its impacts, and the degree of adaptation required. We present a simple conceptual model of anthropogenic CO₂ emissions to highlight the trade off between delay in commencing mitigation, and the strength of mitigation then required to meet specific atmospheric CO₂ stabilization targets. We calculate the effects of alternative emission profiles on atmospheric CO₂ and global temperature change over a millennial timescale using a simple coupled carbon cycle-climate model. For example, if it takes 50 years to transform the energy sector and the maximum rate at which emissions can be reduced is ?2.5% $\text{year}^{-1}$ , delaying action until 2020 would lead to stabilization at 540 ppm. A further 20 year delay would result in a stabilization level of 730 ppm, and a delay until 2060 would mean stabilising at over 1,000 ppm. If stabilization targets are met through delayed action, combined with strong rates of mitigation, the emissions profiles result in transient peaks of atmospheric CO₂ (and potentially temperature) that exceed the stabilization targets. Stabilization at 450 ppm requires maximum mitigation rates of ?3% to ?5% $\text{year}^{-1}$ , and when delay exceeds 2020, transient peaks in excess of 550 ppm occur. Consequently tipping points for certain Earth system components may be transgressed. Avoiding dangerous climate change is more easily achievable if global mitigation action commences as soon as possible. Starting mitigation earlier is also more effective than acting more aggressively once mitigation has begun.     

Late Quaternary environmental and human events at En Gedi, reflected by the geology and archaeology of the Moringa Cave (Dead Sea area, Israel)

The Moringa Cave within Pleistocene sediments in the En Gedi area of the Dead Sea Fault Escarpment contains a sequence of various Pleistocene lacustrine deposits associated with higher-than-today lake levels at the Dead Sea basin. In addition it contains Chalcolithic remains and 5th century BC burials attributed to the Persian period, cemented and covered by Late Holocene travertine flowstone. These deposits represent a chain of Late Pleistocene and Holocene interconnected environmental and human events, echoing broader scale regional and global climate events. A major shift between depositional environments is associated with the rapid fall of Lake Lisan level during the latest Pleistocene. This exposed the sediments, providing for cave formation processes sometime between the latest Pleistocene (ca. 15 ka) and the Middle Holocene (ca. 4500 BC), eventually leading to human use of the cave. The Chalcolithic use of the cave can be related to a relatively moist desert environment, probably related to a shift in the location of the northern boundary of the Saharo-Arabian desert belt. The travertine layer was U-Th dated 2.46 ± 0.10 to 2.10 ± 0.04 ka, in agreement with the archaeological finds from the Persian period. Together with the inner consistency of the dating results, this strongly supports the reliability of the radiometric ages. The 2.46-2.10 ka travertine deposition within the presently dry cave suggests a higher recharge of the Judean Desert aquifer, correlative to a rising Dead Sea towards the end of the 1st millennium BC. This suggests a relatively moist local and regional climate facilitating human habitation of the desert.     

Characteristics of alkenones synthesized by a bloom of emiliania huxleyi in the Bering Sea

We investigated the characteristics of the alkenones produced by a bloom of Emiliania huxleyi in the eastern Bering Sea in 2000. Alkenones were detected in surface waters between 57°N and 63°N, where phosphate concentrations were low and the ammonium/nitrate ratio was high. The total alkenone content (C_37:2, C_37:3, and C_37:4) ranged from 22.0 to 349 μg g⁻¹ in suspended particles and from 0.109 to 1.42 μg g⁻¹ in surface sediments. This suggests that a large proportion of the particulate alkenones synthesized in the surface water rapidly degraded within the water column and/or at the water-sediment interface of the Bering Shelf. The change in the stable carbon isotopic composition (δ¹³C) of C_37:3 alkenone could not be explained only by variation in [CO₂(aq)] in the surface water but also depended on the growth rate of E. huxleyi. The alkenone unsaturation index (U^K′₃₇) was converted into an alkenone “temperature” with three equations [Prahl et al 1988], [Sikes et al 1997] and [Müller et al 1998]; Sikes et al.’s (1997) equation gave the best correlation with the observed sea surface temperature (SST) in the eastern Bering Sea. However, some temperatures estimated by Sikes et al.’s (1997) equation from the U^K′₃₇ varied from the observed SST, possibly because of the rapidly changing rate of alkenone synthesis in the logarithmic growth stage or the low rate of alkenone synthesis when nutrients were limiting. Temperatures estimated from U^K′₃₇ in the surface sediments (6.8-8.2°C) matched the observed SST in September (7-8°C) but differed from the annual average SST of 4 to 5°C, suggesting that most of the alkenone in the eastern Bering Sea was synthesized during limited periods, for instance, in September. The relative amounts of C_37:4 alkenone as proportions of the total alkenones (referred to as C_37:4%) were high, ranging from 18.3 to 41.4%. Low-salinity water (<32 psu) within the study area would have contributed to the high C_37:4% because a negative linear relationship between C_37:4% and salinity was found in this study.