explosive volcanism during the holocene in the upper limay river basin the effects of ashfalls on human societies northern patagonia argentina abstract in order to understand human response to holocene ashfall events tephra layers found in archaeological sites along the upper limay river basin northern patagonia argentina were bracketed with radiocarbon dates and correlated with tephra from a lacustrine sediment core and from outcrops and the archaeological evidence was analyzed a dark tephra associated with seismic activity was identified in the el trebol rockshelter filling interstices between fallen blocks together with remains of human activity and bones of extinct fauna marking a seismo volcanic event occurring between 11 758 and 12 866 cal yr bp this same seismic event affected the cuyin manzano site where the roof collapse made the site uninhabitable for a time a white tephra present in epullan grande epullan chica and traful i caves is correlated with nahuel huapi tephra nht equivalent to laya s rio blanco and rio pereyra members rio pireco formation nht is considered to have been derived from the same eruptive event with dates ranging between ca 1950 2500 cal yr bp a dark tephra from ortega s cave and several tephra from puerto tranquilo i rockshelter ranging between 521 and 2069 cal yr bp show how the mobile hunters gatherers of northern patagonia were able to cope with the changing circumstances

stable isotopes reveal differential patterns of holocene environmental change among tuco tucos rodentia ctenomyidae ctenomys from patagonia abstract to explore the role of past environmental changes in shaping interspecific differences in genetic structure among tuco tucos rodentia ctenomyidae ctenomys from the limay valley of northern patagonia we compared 13c and 18o values for modern and historical tooth enamel from the colonial tuco tuco c sociabilis and the parapatric patagonian tuco tuco c haigi specifically we sought to test the hypothesis that a mid holocene loss of genetic variation in c sociabilis was associated with environmental change for both species the 13c composition of tooth enamel differed significantly 5 1 difference in mean 13c values between modern and holocene 0 5 12 kyr b p samples suggesting marked temporal changes in the vegetation consumed for c sociabilis no statistically significant differences in the 13c composition of teeth were found among individuals from the early middle and late holocene in contrast significant differences in 13c values were found between middle and late holocene samples from c haigi none of our analyses revealed statistically significant differences in 18o values thus although the diets of both study species have changed from the holocene to the present only c haigi experienced significant dietary changes during the holocene we suggest that the ecologically more specialized c sociabilis continued to consume the same food resources even as changing holocene conditions caused preferred habitats to disappear in contrast the more generalized c haigi altered its diet allowing populations of this species to remain widespread despite changing holocene environments

importance of organic fe complexing ligands in the mississippi river plume abstract as part of a larger program focused on understanding the biogeochemistry of large river plumes we participated in two expeditions during 2000 to sample the mississippi river plume surface water samples were collected using a trace metal clean towed fish and analyzed for total dissolved fe organic fe complexing ligands and their associated conditional stability constants the ligands in the river plume have conditional stability constants log k fel between 10 5 and 12 3 with an average of 11 2 and standard deviation of 0 6 very similar to ligands found in the open ocean comparison of high flow and low flow regimes indicates that variability in flow may be the main cause of the variability in fe concentrations in the plume the organic fe complexing ligands are in greatest excess during a time of higher flow these ligands are responsible for maintaining very high 5 nm fe concentrations throughout the plume due to complexation with these organic ligands the concentration of fe remains above the fe hydroxide solubility level until a salinity above 35 is reached where there appears to be a sink for fe in the less productive waters therefore fe is transported a great distance from the river source and is available for biological utilization in the coastal zone

carbon export and cycling by the yukon tanana and porcupine rivers alaska 2001 2005 1 loads and yields of dissolved and particulate organic and inorganic carbon doc poc dic pic were measured and modeled at three locations on the yukon river yr and on the tanana and porcupine rivers tr pr in alaska during 2001 2005 total yr carbon export averaged 7 8 tg c yr 1 30 as oc and 70 as ic total c yields 0 39 1 03 mol c m 2 yr 1 were proportional to water yields 139 356 mm yr 1 r2 0 84 at all locations summer doc had an aged component fraction modern fm 0 94 0 97 except in the permafrost wetland dominated pr where doc was modern poc had fm 0 63 0 70 doc had high concentration high aromaticity and high hydrophobic content in spring and low concentration low aromaticity and high hydrophilic content in winter about half of annual doc export occurred during spring dic concentration and isotopic composition were strongly affected by dissolution of suspended carbonates in glacial meltwater during summer

recent volcanism in the puyehue cordon caulle region southern andes chile 40 5 s petrogenesis of evolved lavas

holocene tephra succession of puyehue cordon caulle and antillanca casablanca volcanic complexes southern andes 40 41 s abstract puyehue cordon caulle and antillanca volcanic complexes are two of at least 50 active frontal arc volcanoes that define the 1400 km long southern volcanic zone of chile holocene tephra deposits in chile and argentina 40 41 s up to 100 km east of these volcanoes comprise at least five voluminous 1 to 8 km 3 pyroclastic fall layers that preceded several recently deposited cordon caulle pumice fallouts field observations of proximal medium and distal facies of the deposits in conjunction with geochronology and geochemistry of the volcanic complexes indicate that three fall layers are derived from puyehue volcano puyehue 1 and 2 and mil hojas whereas two are sourced from the antillanca complex playas blanca negra and nahuel huapi tephra 20 km to the south the oldest tephra calibrated 14 c age 10 49 0 12 ka 2 found only at medium distal facies is deposited directly on granitic moraine boulders and consists of deeply weathered orange dacitic pumice lapilli the next prominent tephra at 7 ka comprises dacitic pumice and its age is equivalent to a rhyodacitic dome exposed in the puyehue summit crater above these deposits there are phases of a complex eruption consisting of a conspicuous compositionally zoned tephra it also comprises a pyroclastic density current together with lithic rich and scoriaceous fallout deposits mineralogical geochemical and sr isotope evidence plus the isopach maps confirm that this sequence of eruptive events is sourced from antillanca at 1932 68 yrbp the total volume of this eruptive sequence exceeds 8 km 3 making it the largest holocene eruption from either volcanic complex this eruption was likely responsible for the destruction of an ancestral antillanca volcano and the formation of a 4 5 km diameter caldera a distinctive younger unit in the region is a voluminous rhyodacitic pumice fall calibrated 14 c age 1 11 0 07 ka above which a series of several alternating dark lithic and pumice lapilli beds accumulated correlation with proximal deposits indicates that the 1 11 ka eruption was derived from puyehue and destroyed 3 km 3 of rhyodacitic domes at this volcano summit historic explosive activity at the nascent casablanca volcano and along cordon caulle including the 2011 2012 eruption 1 km 3 of uncompacted pumice the largest from this fissural zone emphasizes an increased risk for volcanic hazards in central chile and argentina

impact of the 1960 major subduction earthquake in northern patagonia chile argentina abstract the recent sedimentation processes in four contrasting lacustrine and marine basins of northern patagonia are documented by high resolution seismic reflection profiling and short cores at selected sites in deep lacustrine basins the regional correlation of the cores is provided by the combination of 137cs dating in lakes puyehue chile and frias argentina and by the identification of cordon caulle 1921 22 and 1960 tephras in lakes puyehue and nahuel huapi argentina and in their catchment areas this event stratigraphy allows correlation of the formation of striking sedimentary events in these basins with the consequences of the may june 1960 earthquakes and the induced cordon caulle eruption along the liquine ofqui fault zone lofz in the andes while this catastrophe induced a major hyperpycnal flood deposit of ca 3 106 m3 in the proximal basin of lago puyehue it only triggered an unusual organic rich layer in the proximal basin of lago frias as well as destructive waves and a large sub aqueous slide in the distal basin of lago nahuel huapi a very recent mega turbidite in the two distal basins of reloncavi fjord located close to the lofz suggests that 1960 co seismic movements in this area may have triggered the remobilization of ca 187 106 m3 of marine sediments

northern latitude chemical weathering rates clues from the mackenzie river basin canada abstract the main scope of this study is to investigate parameters controlling chemical weathering rates for a large river system submitted to subarctic climate more than 110 river water samples from the mackenzie river system northern canada have been sampled and analyzed for major and trace elements and sr isotopic ratios in the dissolved phase the three main morphological units are reflected in water chemistry rivers from the canadian shield are very dilute dominated by silicate weathering millot et al 2002 whereas the rivers of the rocky and mackenzie mountains as well as the rivers of the sedimentary interior platform are dominated by carbonate weathering and are so 4 rich compared to the rivers of the mackenzie and rocky mountains the rivers of the interior plains are organic silica and na rich and constitute the dominant input term to the mackenzie river mainstream rivers of the canadian shield area do not significantly contribute to the mackenzie river system using elemental ratios and sr isotopic ratios a mathematical inversion procedure is presented that distinguishes between solutes derived from silicate weathering and solutes derived from carbonate weathering carbonate weathering rates are mostly controlled by runoff which is higher in the mountainous part of the mackenzie basin these rates are comparable to the carbonate weathering rates of warmer areas of the world it is possible that part of the carbonate weathering is controlled by sulfide oxidative weathering but its extent remains difficult to assess conversely to what was stated by edmond and huh 1997 overall silicate weathering rates in the mackenzie basin are low ranging from 0 13 to 4 3 tons km 2 yr na k ca mg and confirm the negative action of temperature on silicate weathering rates for river basins in cold climates in contrast to what has been observed in other large river systems such as the amazon and ganges rivers silicate weathering rates appear 3 to 4 times more elevated in the plains than in the mountainous headwaters this contradicts the raymo hypothesis raymo and ruddiman 1992 isotopic characterization of suspended material clearly shows that the higher weathering rates reported for the plains are not due to the weathering of fine sediments produced in the mountains e g by glaciers and deposited in the plains rather the relatively high chemical denudation rates in the plains are attributed to lithology uncompacted shales high mechanical denudation and the abundance of soil organic matter derived from incomplete degradation and promoting crystal lattice degradation by element complexation the three to fourfold factor of chemical weathering enhancement between the plains and mountains is similar to the fourfold factor of enhancement found by moulton et al 2000 between unvegetated and vegetated watershed this study confirms the negative action of temperature on silicate weathering for cold climate but shows that additional factors such as organic matter associated with northern watersheds are able to counteract the effect of temperature this acceleration by a factor of 4 in the plains is equivalent to a 6 c increase in temperature

iron in the japan sea and its implications for the physical processes in deep water 1 labile dissolved fe dfe 0 22 m fraction and total dissolvable fe tdfe unfiltered were examined throughout the water column of the northeastern japan basin and mid eastern yamato basin basin regions in the japan sea we observed extremely high vertically integrated tdfe inventories and low nutrient concentrations at the surface water in both basins probably resulting from high atmospheric fe input to nutrient depleted surface water dfe in both basins was characterized by mid depth maxima and below that a slight decrease with depth in deep water and uniform concentration in bottom water in the japan basin surprisingly tdfe concentrations in bottom water were lower than those in deep water resulting from the injection of new bottom water on the contrary tdfe concentrations in bottom water of the yamato basin increased gradually with depth and were higher than those in deep water probably due to the supply of iron during the horizontal transport of the bottom water in the japan basin towards the yamato basin the tdfe with a high percentage of labile particulate fe tdfe minus dfe may have important implications for understanding the physical processes of bottom water in the japan sea

the plume of the yukon river in relation to the oceanography of the bering sea abstract physical and biological oceanography of the northern bering sea including the plume of the yukon river were studied using satellite data in conjunction with shipboard measurements the satellite data recorded by the noaa very high resolution radiometer vhrr and advanced very high resolution radiometer avhrr and the landsat multispectral scanner mss and thematic mapper tm sensors were used to detect sea surface temperatures and suspended sediments shipboard measurements of temperature salinity and chlorophyll were acquired by the inner shelf transfer and recycling ishtar project and were compared to digitally enhanced and archived satellite images sea surface temperatures derived from satellite data were generally higher than field measurements this difference was the likely result of microscale stratification of the water column although other factors could have been involved also satellite data confirmed the known distribution of water masses in the region coachman et al 1975 upwelled water of oceanic origin anadyr water was visible as a cold surface plume running from anadyr strait north through western bering strait in the east warm alaskan coastal water was prominent along the alaskan coast including norton sound and the southeastern chukchi sea areal patterns of temperature salinity and phytoplankton distribution determined from field measurements agreed reasonably well with patterns of water mass distribution obtained from satellite images archived satellite images 1974 1978 were used to investigate the variability of the distribution of sea surface temperature and of the turbid plume of the yukon river alaskan coastal water acw first warms near the coast in june and the process extends seaward as summer progresses turbid water associated with discharge of the yukon river progresses in the same fashion extending northward across the entrance to norton sound maximum extent of the plume occurs in october anadyr water flows north through anadyr strait past st lawrence island its extent is variable depending on mesoscale pressure and local wind fields

late quaternary tephrostratigraphy of southern chile and argentina abstract the southern and austral volcanic zones of the andes comprise 74 volcanic centres with known post glacial activity at least 21 of these have had one or more large explosive eruptions in the late quaternary dispersing tephra over vast areas these tephra layers therefore have great potential as tephrochronological marker horizons in palaeoenvironmental studies in southern chile and argentina a region that is particularly useful to study climate dynamics of the southern hemisphere however to date tephrochronology has rarely been fully utilised in this region as a correlation and dating tool here we review the existing post glacial tephrostratigraphic record of the southern and austral volcanic zones and compile a database of known occurrences of tephra from these volcanoes in ice and lacustrine marine peat and cave sediment records we address the inconsistencies in and revisions of the tephrostratigraphies presented in prior literature and discuss the challenges in correlating tephras and the limitations of the tephrostratigraphic record in this area this study highlights the many gaps that still exist in our knowledge of the eruptive histories of these volcanoes but also reveals the largely under utilised potential of tephra as a correlation tool in this region this is exemplified by the severe lack of adequate geochemical analysis of tephra layers preserved in many lacustrine and peat sediment sections which are particularly important tephrostratigraphic records in southern chile and argentina due to the paucity of surface preservation

seismic attenuation structure of the top half of the inner core beneath the northeastern pacific 1 we present a 1 d seismic attenuation model of the top 600 km of the inner core beneath the northeastern pacific we employ a waveform inversion method for attenuation parameters based on simulated annealing that enables us to analyze complicated waveforms contaminated by phase overlaps the method also allows analysis of pkp df in distant ranges where only single core phase is present the resultant data set is about five times larger compared with our previous study the inverted model shows a gradual increase of the attenuation from the icb to 200 250 km depth that coincides with a gradual increase of the p wave velocity in the same depth range the attenuation decreases from this depth range and thus shows a peak around 200 250 km a possible relation of these observations to the growth process of the inner core in the western hemisphere oftheinnercoreisdiscussed citation iritani r n takeuchi and h kawakatsu 2010 seismic attenuation structure of the top half of the inner core beneath the northeastern pacific geophys res lett 37 l19303 doi 10 1029 2010gl044053

distribution of atlantic and pacific waters in the upper arctic ocean implications for circulation the atlantic and pacific oceans provide source waters for the arctic ocean that can be distinguished by their differing nitrate and phosphate concentration relationships using these relationships we estimate the amount of atlantic and pacific waters in the surface layer top 30 m of the arctic ocean atlantic source water is dominant in most of the eurasian basin and is present in significant amounts in the makarov basin north of the east siberian sea pacific source water is dominant in most of the canadian basin and is present in significant amounts in the amundsen basin north of greenland we deduce circulation patterns from the distributions of atlantic and pacific source waters in the surface layer of the arctic ocean and conclude that the flow within the surface layer differs from ice drift along the north american and european boundaries of the polar basin

iron nutrients and humic type fluorescent dissolved organic matter in the northern bering sea shelf bering strait and chukchi sea 1 the concentrations of dissolved fe dfe total dissolvable fe humic type fluorescence intensity humic f intensity as humic type fluorescent dissolved organic matter humic type fdom and nutrients were determined vertically in the northern bering sea shelf yukon river estuary region and st lawrence island polynya region the eastern chukchi sea shelf chukchi shelf polynya region and the central bering strait during 30 july to 12 august 2007 and 30 june to 12 july 2008 in early july 2008 with a nearly peak river water discharge period remarkably high humic f intensity and high dfe were found with a decrease in salinity s 26 30 in the surface water of the yukon river estuarine region these results suggest that riverine humic type fdom is responsible for dfe in coastal waters with natural organic ligand complexation of fe iii apparently playing an important role in iron transport to the northern bering sea shelf in the st lawrence island and chukchi shelf polynya regions cold dense waters t 1 5 to 1 7 c and 26 2 to 26 7 were found in deep and bottom waters the cold waters are characterized by high nutrient and iron concentrations and high humic f intensity probably resulting from two main processes brine rejection during sea ice formation and transport across the sediment water interface during early diagenesis in the central bering strait the waters are vertically uniform with high salinity s 32 5 and high nutrient and iron concentrations these waters appear to originate from the outer shelf of the bering sea and are not affected by denitrification occurring in bottom sediments of the bering shelf

postglacial vegetation climate and fire history along the east side of the andes lat 41 42 5 s argentina abstract the history of the low elevation forest and forest steppe ecotone on the east side of the andes is revealed in pollen and charcoal records obtained from mid latitude lakes prior to 15 000 cal yr bp the vegetation was characterized by steppe vegetation with isolated stands of nothofagus the climate was generally dry and the sparse vegetation apparently lacked sufficient fuels to burn extensively after 15 000 cal yr bp a mixture of nothofagus forest and shrubland steppe developed fire activity increased between 13 250 and 11 400 cal yr bp contemporaneous with a regionally defined cold dry period huelmo mascardi cold reversal the early holocene period was characterized by an open nothofagus forest shrubland mosaic and fire frequency was high in dry sites and low in wet sites the data suggest a sharp decrease in moisture eastward from the andes a shift to a surface fire regime occurred at 7500 cal yr bp at the wet site and at 4400 cal yr bp at the dry site preceding the expansion of austrocedrus by 1000 1500 yr the spread of austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late holocene the change to a surface fire regime is consistent with increased interannual climate variability and the onset or strengthening of enso the present day mixed forest dominated by nothofagus and austrocedrus was established in the last few millennia

the continental margin is a key source of iron to the hnlc north pacific ocean here we show that labile particulate iron and manganese concentrations in the upper 500m of the western subarctic pacific an iron limited high nutrient low chlorophyll hnlc region have prominent subsurface maxima between 100 200 m reaching 3 nm and 600 pm respectively the subsurface concentration maxima in particulate fe are characterized by a more reduced oxidation state suggesting a source from primary volcagenic minerals such as from the kuril kamchatka margin the systematics of these profiles suggest a consistently strong lateral advection of labile mn and fe from redox mobilized labile sources at the continental shelf supplemented by a more variable source of fe from the upper continental slope this subsurface supply of iron from the continental margin is shallow enough to be accessible to the surface through winter upwelling and vertical mixing and is likely a key source of bioavailable fe to the hnlc north pacific

synchronisation of sedimentary records using tephra a postglacial tephrochronological model for the chilean lake district abstract well characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives the use of tephra as a chronological tool is however still widely underutilised in southern chile and argentina in this study we develop a postglacial tephrochronological model for the chilean lake district ca 38 to 42 s by integrating terrestrial and lacustrine records tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout by combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date we present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental palaeoclimatological and palaeoseismological purposes most volcanoes in the studied segment of the southern volcanic zone between llaima and calbuco have produced at least one regional marker deposit resulting from a large explosive eruption magnitude 4 some of which now have a significantly improved age estimate e g the 10 5 ka llaima pumice eruption from llaima volcano others including several units from puyehue cordon caulle are newly described here we also find tephra related to the cha1 eruption from chaiten volcano in lake sediments up to 400 km north from source several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives our chronological model suggests three distinct phases of eruptive activity impacting the area with an early to mid holocene period of relative quiescence extending our tephrochronological framework further south into patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial

implications for paleomobility studies of the effects of quaternary volcanism on bioavailable strontium a test case in north patagonia argentina abstract strontium isotopes 87sr 86sr are used as geochemical tracers for paleomobility studies because they display predictable and stable patterns in ecosystems primarily controlled by the underlying geological regimes while bedrock geology is stable over thousands of years geomorphological processes can influence the 87sr 86sr in ecosystems over archeologically relevant timescales among these geomorphological processes the deposition and reworking of volcanic sediments over quaternary timescales are little studied but could be an important control of 87sr 86sr variations in many archeological regions north patagonia is a key archeological region to address animal and human movements and an ideal location to test the influence of quaternary volcanism on 87sr 86sr variation as it is located downwind of major volcanic centers in this study we aim to assess the main environmental and geological controls of bioavailable 87sr 86sr and to build a high resolution isoscape using a machine learning regression framework for forthcoming paleomobility studies we sampled several locations and analyzed different types of samples n 94 the ratios show a limited range of variation which is not related to the bedrock geology rather bioavailable 87sr 86sr variations display a progressive increase going eastward away from the andes following dust aerosol deposition and elevation variations r2 0 71 rmse 0 00041 we argue that this trend relates to the deposition and reworking of unradiogenic volcanic sediments by aeolian fluvial and glacial erosion during the quaternary as most of this sediment reworking occurred during glacial periods the current bioavailable 87sr 86sr variations across the study area likely represent a long term average that varied little during the holocene consequently our isoscape provides a solid base for holocene paleomobility studies in north patagonia and underlines the importance of quaternary volcanism processes for interpreting 87sr 86sr data in paleomobility studies in volcanic regions

tephrostratigraphy of a deep sea sediment sequence off the south chilean margin new insight into the hudson volcanic activity since the last glacial period abstract a detailed tephrostratigraphy study supported by accelerator mass spectrometry ams 14 c dating stable isotope 18 o and geochemical analyses has been carried out from a deep sea core collected offshore the chonos taitao archipelago southern chile close to the main andean volcanic centres of the southern volcanic zone svz counting of glass shards in the drill core revealed the occurrence of twelve ash layers over the last 22 ka bp their provenance was determined by comparing the 14 c age and major element geochemistry of glass shards with tephra recovered in terrestrial fallout deposits from the southern south volcanic zone ssvz four cryptotephra layers in the uppermost part of the studied core 1 9 16 1 ka bp can be confidently correlated to the corresponding cerro hudson volcanic eruptive events hw7 hw3 hw2 and hw1 for the eight remaining tephra layers deposited between 16 1 ka and 20 ka bp no direct correlation is possible with onland volcanic products however the major element geochemistry of the glass shards strongly suggests the cerro hudson as the volcanic source for these tephra on the other hand the lack of two of the largest hudson holocene eruptions 3 6 and 7 5 ka bp in the uppermost part of deep sea core may be linked to changes in the position of the southern westerlies wind belt sww as a response to the climate variability in northern patagonia during the holocene

human subsistence and environmental stability during the last 2200 years in epullan chica cave northwestern patagonia argentina a perspective from the zooarchaeological record abstract the faunal content from epullan chica archaeological site ech thereafter a small cave located in northwestern patagonia argentina was studied from taphonomic and paleoenvironmental points of view this cave is placed in the ecotone between monte desert and the patagonian steppe in the middle limay river basin was occupied since the end of the late holocene zooarchaeological evidence retrieved from ech is diverse including fresh water mollusk shells diplodon chilensis eggshell and bone fragments of rheidae bones and teeth of large lama guanicoe medium e g chaetophractus villosus conepatus chinga and micro sized several species of sigmodontine and caviomorph rodents mammals most of the recorded taxa were the result of human exploitation however owl pellets preserved in the sediments and other taphonomic signatures clearly indicate that avian predators are responsible for part of the micromammal record micromammal abundances during the last 2 2 ka bp are suggestive of a relative environmental stability around ech at least until the beginning of the last century in this sense late holocene landscapes were dominated by open steppe areas and large rocky outcrops with minor changes in humidity and temperature during this period

holocene stability and recent dramatic changes in micromammalian communities of northwestern patagonia abstract micromammalian fossil sequences of two classical archaeological sites cueva traful i cti and cueva epullan grande ll both in the province of neuquen northwestern patagonia argentina were studied these sequences cover the entire holocene with assemblages mainly composed by rodent and marsupial remains accumulated by owls both sites are located near the ecotone fringes of the major vegetation units currently present in patagonia forest patagonian steppe cti and patagonian steppe monte desert ll comparisons between fossil and recent owl pellet samples in a regional approach indicated that micromammal communities remained stable over the major part of the last 10 ka with some minor expansion of desert adapted taxa to the west since the middle holocene and a dramatic change in the assemblages during the last 100 150 years two alternative but not contradictory explanations can be presented to interpret the stable period first climate environmental changes were not of sufficient magnitude to produce a clear reorganization of micromammal communities forest did not invade cti surroundings and monte did not reach around ll second the basic structure of northwestern patagonian steppe micromammal communities was resilient to minor or moderate climate environmental changes the strong shift depicted in the studied sequences in very recent times 100 150 years can be linked to human impact on the regional environment this change involves a deep restructuration of small mammal communities ranging from virtual extirpations of some species to dramatic increments of opportunistic taxa a century of sheep overgrazing coupled with the introduction of exotic shrubs was the main factor driving the configuration of present small mammal assemblages in northwestern patagonia

vertical distributions of iron iii hydroxide solubility and dissolved iron in the northwestern north pacific ocean detailed vertical distributions of fe iii hydroxide solubilities and dissolved fe concentrations which are strongly related to the concentration and affinity of natural organic fe iii chelators in seawater were measured at three typical stations in the northwestern north pacific ocean iron iii hydroxide solubility in the surface mixed layer was generally high and variable 0 3 2 4 nm corresponding with the depth of high chlorophyll a concentrations and the solubility minima 0 2 0 4 nm occurred at 75 125 m depth the vertical profiles of fe iii hydroxide solubility in mid depth and deep waters are characterized by mid depth maxima 0 6 0 7 nm and subsequently a slight decrease to 0 4 0 5 nm with depth which is markedly similar to nutrient and dissolved fe depth profiles the solubility profiles reveal that dissolved fe concentrations in deep ocean waters are controlled primarily by the fe iii complexation with natural organic ligands which were released through the oxidative decomposition and transformation of biogenic organic matter in mid depth and deep waters