The potential of synthetic aperture radar interferometry for assessing meltwater lake dynamics on Antarctic ice shelves

Surface meltwater drains on several Antarctic ice shelves, resulting in surface and sub-surface lakes that are potentially critical for the ice shelf collapse. Despite these phenomena, our understanding and assessment of the drainage and refreezing of these lakes is limited, mainly due to lack of field observations and to the limitations of optical satellite imagery during polar night and in cloudy conditions. This paper explores the potential of backscatter intensity and of interferometric coherence and phase from synthetic aperture radar (SAR) imagery as an alternative to assess the dynamics of meltwater lakes. In four case study regions over Amery and Roi Baudouin ice shelves, East Antarctica, we examine spatial and temporal variations in SAR backscatter intensity and interferometric (InSAR) coherence and phase over several lakes derived from Sentinel-1A/B C-band SAR imagery. Throughout the year, the lakes are observed in a completely frozen state, in a partially frozen state with a floating ice lid and as open-water lakes. Our analysis reveals that the meltwater lake delineation is challenging during the melting period when the contrast between melting snow and lakes is indistinguishable. Despite this finding, we show using a combination of backscatter and InSAR observations that lake dynamics can be effectively captured during other non-summertime months. Moreover, our findings highlight the utility of InSAR-based observations for discriminating between refrozen ice and sub-surface meltwater and indicate the potential for phase-based detection and monitoring of rapid meltwater drainage events. The potential of this technique to monitor these meltwater change events is, however, strongly determined by the satellite revisit interval and potential changes in scattering properties due to snowfall or melt events.

Geochemical and sedimentary constraints on the formation of the Venta Micena early Pleistocene site (Guadix-Baza Basin, Spain)

Despite the paleontological relevance of the terrestrial Early Pleistocene Venta Micena bonebed (Baza Basin, Spain), it lacks a comprehensive geochemical/sedimentological study. Here, we demonstrate that the 1.5-m-thick Venta Micena limestone formed in a relatively small freshwater wetland/pond located at the periphery of the large saline Baza paleolake. Two microfacies are observed, with high and low contents of invertebrate fossils, and which originated in the centre and margin of the wetland, respectively. X-ray diffraction (XRD) mineralogy and paleohydrological characterization based on ostracod and bulk-rock geochemistry (δ13C and δ18O) indicate that the limestone reflects a general lowstand of the Baza lake, permitting the differentiation of freshwater wetlands that were fed by adjacent sources. Conversely, during highstands, the Baza lake flooded the Venta Micena area and the freshwater fauna was replaced by a saline one. Bulk-rock isotopic data indicate that the lower interval C1 of the limestone (bone-rich in marginal settings) displays general negative values, while the upper interval C2 (bone free) displays less negative values. The bones of predated mammals accumulated in the marginal areas, which were flooded and buried by recurring water-table fluctuations. Lake dynamics played a critical role in bone accumulation, which was previously considered as representing a hyena den.

The potential of InSAR for assessing meltwater lake dynamics on Antarctic ice shelves

Surface meltwater drains on several Antarctic ice shelves, resulting in surface and sub-surface lakes that are potentially critical for the ice shelf collapse. Yet, our understanding and assessment of the drainage or refreezing of these lakes is limited, mainly due to lack of field observations and to the limitations of optical satellite imagery. Therefore, this paper explores the potential of backscatter intensity and of interferometric coherence and phase from C-band synthetic aperture radar (SAR) imagery as an alternative to assess the dynamics of meltwater lakes. In two case studies over Amery and Roi Baudouin ice shelves, we analyse i) the spatial and ii) the temporal variations of SAR backscatter intensity with iii) coherence and iv) interferogram phase (InSAR) patterns detected by Sentinel-1 data over multiple meltwater lakes. Throughout the year the lakes are observed in completely frozen state, in partially frozen state with a floating ice lid, and as open water lakes. The analysis reveals that the meltwater lake delineation is challenging during the melting period when the contrast between melting snow and lakes is confounded. On the other hand, it shows that the lake dynamics can be effectively captured during the refreezing process and the winter season by combining backscatter and InSAR information. In particular, the InSAR coherence and interferogram phase information are deemed essential throughout this whole period to distinguish between refrozen ice and subsurface meltwater. Additionally, the results provide significant evidence on the potential of the interferogram fringe patterns to detect and characterise instant events, such as lake drainage events over ice shelves. The potential of this technique to monitor these meltwater change events is however strongly determined by the satellite revisit interval and potential changes in scattering properties due to snowfall or melt events.

How do recent depositional processes changes in high altitude Pyrenean lakes compare with Late Holocene variability? The case of Montmalús Lake (Principality of Andorra, Eastern Pyrenees)

<p>High mountain lakes are pristine ecosystems that archive in their sediments high-resolution records of watershed and lake evolution. Understanding how they have responded to Holocene climate fluctuations and anthropic impacts provides essential information to put into a historical context the magnitude and unique features of the current global change.</p><p>The REPLIM project funded by the Interreg program (POCTEFA 2014-2020) has implemented a network of lakes in Spain, France and Andorra to study current and past climate, environmental and anthropic changes in lakes. In August 2017, eight short sediment cores were recovered in Montmalús Lake (2433 m a.s.l., Andorra). We present a paleolimnological reconstruction based on sedimentary facies, chemical (main and trace element) analyses and δ<sup>13</sup>C and δ<sup>15</sup>N of bulk organic matter. We developed a robust age model based on <sup>210</sup>Pb, <sup>137</sup>Cs and <sup>14</sup>C dating for the last 2000 years. Moreover, modern lake dynamics have been characterized with continuous water temperature measurements at various depths, periodical water sampling and sediment traps.</p><p>The results show large depositional fluctuations in the lake dynamics, especially during the Little Ice Age, with changes in organic matter accumulation, bioproductivity and sources and increased sediment delivery. Sedimentological and geochemical indicators point to the onset of high human impact in the landscape around the 11<sup>th</sup> century. Also, medieval mining and metallurgic activity from 11<sup>th</sup> to 14<sup>th</sup> centuries increased Pb deposition.</p><p>The 20<sup>th</sup> century is characterized by a recovery in organic accumulation and bioproductivity rates. Heavy metal deposition also increased during the late 20<sup>th</sup> century and started to decrease in the 2010’s decade, without reaching the background values</p><p>This study provides the first data on recent changes in lakes from the Principality of Andorra. The results highlight the uniqueness of current global change impacts in alpine lakes and underline the interplay of Great Acceleration and Global Warming processes in these fragile and sensitive depositional systems.</p>