The influence of hydrosol nonsphericity on the polarization characteristics of light under water is investigated by combining accurate single-scattering models for randomly oriented spheroidal scatterers with a radiative transfer model that employs Stokes formalism and considers refraction of direct unpolarized solar radiation and 100% linearly polarized radiation at the air–water interface followed by single scattering. Variations in what we call the “linear polarization phase function” (the degree of linear polarization as a function of scattering angle and the angle of linear polarization as a function of scattering angle) are examined for a wide range of spheroid aspect ratios and complex refractive indices of hydrosols. Implications for polarization-sensitive marine organisms and for remote sensing of the marine environment are discussed.

A new paper by our graduate student Yonathan Keinan (with co-authors Miryam Bar-Matthews, Avner Ayalon, and Tami Zilberman (Geological Survey of Israel) and Amotz Agnon and Amos Frumkin (the Institute of Earth Sciences, Hebrew University) analyzes speleothems from the Zalmon Cave, located at the current rain shadow zone in the northern Upper Jordan River Valley, Israel. The speleothm data indicate a humid phase with increased precipitation during the last glacial, borne out from a change in the storm track over the eastern Mediterranean. Zalmon precipitation was probably somewhat lower than in the higher altitudes in the Upper Galilee and the central Judean Mountains of Israel. This conclusion supports earlier views of increased precipitation in the headwaters of the Dead Sea basin during that interval. Furthermore, the new paper indicates that unlike the spelothems from the Ma'ale Efrayim cave, which are impacted by the rain shadow of the Samaria Mountains, and show no speleothm activity during the interglacial and possibly dry vadose zone, these Zalmon Cave speleothm data indicate permanent water in the vadose zone during both glacial and interglacials. 

The isotopic composition of Thorium in marine sediment cores from east of Africa shows that Sahara dust fluxes decreased by at least 50% between the last deglaciation and the mid Holocene, while the Northern Sector of the Red Sea remained unchanged. This constrains the African Humid Period impact to have extended up to ca. 22°N, across a more limited region than previously thought. 

 

 

 

Light polarization characteristics, i. e., degree of linear polarization (DoLP) and angle of linear polarization (AoLP), were documented in the depth of the littoral cave system of Rosh HaNikra on the northern Mediterranean shore of Israel (33° 5′ 35.24″ N, 35° 6′ 17.16″ E), based on light intensity sampled through polarizing filters at different hours of the day on different days of the year. This is the first study to investigate the state of light polarization in such a unique habitat in which photosynthetic organisms, such as cyanobacteria, microalgae, and macroalgae thrive. Such organisms play an essential ecological role as the energy base for the cave's fauna. Using these two methods, we found unique winter polarization characteristics within the cave, including high values of DoLP in the morning and at noon, reaching 50%, and nearly constant AoLP throughout the day. Given the low levels of light intensity that typically exist within the cave in the winter months, the relatively high DoLP and the nearly constant AoLP throughout the day may play a significant role in improving the ability of photosynthetic organisms within the cave to harvest light by orienting their light-harvesting receptors with respect to the AoLP. Using the polarization photograph analysis method, we were able to determine the polarization characteristics originating from the sky, reflection off of (including refraction into followed by refraction out of) the far sea surface, and reflection off of the cave wall separately. The maximum DoLP values originating from the sky, far ocean, and cave walls were found to be 27, 50, and 35%, respectively. The lowest daily variation in AoLP was that of light reflecting off the cave walls. The present study lays the foundation for any subsequent study of the role of light polarization in the distribution of the algal flora on the cave walls in and out of the water in the Rosh HaNikra cave and in sea caves in general.

 

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