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Natural lakes are thought to be globally important sources of greenhouse gases (CO 2 , CH 4 , and N 2 O) to the atmosphere although nearly no data have been previously reported from Africa. We collected CO 2 , CH 4 , and N 2 O data in 24 African lakes that accounted for 49% of total lacustrine surface area of the African continent and covered a wide range of morphology and productivity. The surface water concentrations of dissolved CO 2 were much lower than values attributed in current literature to tropical lakes and lower than in boreal systems because of a higher productivity. In contrast, surface water–dissolved CH 4 concentrations were generally higher than in boreal systems. The lowest CO 2 and the highest CH 4 concentrations were observed in the more shallow and productive lakes. Emissions of CO 2 may likely have been substantially overestimated by a factor between 9 and 18 in African lakes and between 6 and 26 in pan-tropical lakes.

We present here a commentary essay on the challenges and perspectives on the recruitment and retention of Malagasy women in STEM (Science, Technology, Engineering, and Mathematics). We base the essay on our personal experience as Malagasy women scientists and support the arguments with the responses of 60 Malagasy women scientists to an online survey that was shared among Malagasy scientists. We identified that the main causes for a limited number of Malagasy women in STEM relied upon traditional expectations and heavily ingrained cultural values. The situation becomes more complicated when accounting for familial constraints, financial constraints, and communication gaps between students and professors. This is obviously an unfortunate situation; however, there have been bridging-gap initiatives— financial, awareness to family pressure, and knowledge difference between senior and junior scientists—that should be continued to provide support to promising students and enhance STEM education as a tool for development in Madagascar. Cet essai discute les défis et les perspectives dans le recrutement et le maintien des femmes citoyennes de Madagascar dans les sciences, la technologie, l’ingénierie et les mathématiques (STIM). Les arguments présentés sont basés sur nos expériences personnelles en tant que femmes scientifiques nées et ayant grandies à Madagascar. Ces arguments sont soutenus par des informations issues d’un sondage effectué en ligne auprès de 60 femmes scientifiques, s’identifiant elles-mêmes originaires de Madagascar. Les raisons pour lesquelles ces femmes sont peu nombreuses à participer activement dans les domaines des STIM à Madagascar sont liées à des contraintes traditionnelles et à des valeurs culturelles fortement ancrées depuis leur enfance. La situation se complique lorsqu’on tient compte des contraintes financières et des écarts de communication entre étudiants et professeurs. Si cette situation est évidemment regrettable, il existe cependant des initiatives visant à combler le fossé qui sont financières, ou qui s’inscrivent dans la sensibilisation à la pression familiale et à la différence de connaissances entre les scientifiques seniors et juniors. Ces encouragements devraient être poursuivis afin d'apporter un soutien aux étudiantes prometteuses et d'améliorer l'enseignement des femmes en STIM qui sont importantes pour le développement de Madagascar.

The 8.2 ka event is a well-known cooling event in the Northern Hemisphere, but is poorly understood in Madagascar. Here, we compare paleoclimate data and outputs from paleoclimate simulations to better understand it. Records from Madagascar suggest two distinct sub-events (8.3 ka and 8.2 ka), that seem to correlate with records from northern high latitude. This could indicate causal relationships via changes in the Atlantic Meridional Overturning Circulation (AMOC) with changes in moisture source's δ18O, and changes in the mean position of the Inter-Tropical Convergence Zone (ITCZ), as climate modelling suggests. These two sub-events are also apparent in other terrestrial records, but the climatic signals are different. The prominent 8.2 ka sub-event records a clear antiphase relationship between the northern and southern hemisphere monsoons, whereas such relationship is less evident during the first 8.3 ka sub-event. Data–model comparison have also shown a mismatch between the paleoclimate data and the model outputs, the causes of which are more or less understood and may lie in the proxies, in the model, or in both data and model. Knowing that paleoclimate proxies and climate models produce different sets of variables, further research is needed to improve the data–model comparison approach, so that both paleoclimate data and paleoclimate models will better predict the likely climate status of a region during a specified time in the past with minimal uncertainties.

Caves are an excellent natural laboratory for understanding the transfer processes of the region’s environmental signals to speleothems. At least eight speleothems have produced high resolution paleoclimate and paleoenvironment records from Anjohibe Cave, NW Madagascar. However, due to the remote and difficult access to many caves in Madagascar, no studies have yet been done to understand the transfer of climate and environmental changes of the region to the cave. This is the first monitoring study to understand the linkage between regional climatology and various responses in Anjohibe Cave. We monitored (1) the drip water pH, TDS, EC, temperature, δ13CDIC, δ18Ow, δ2Hw, and elemental (Ca, Mg, Sr) composition, and (2) the cave atmosphere pCO2, relative humidity and temperature. Results show that air-to-air transfer is fast, and the internal parameters closely vary with the regional climatology. In contrast, rainfall to drip signal transfer is not immediate, and it can take few months to one season for the signals to be detected in the drip water due to the “epikarst storage effect”. The deposition of CaCO3 is inferred to occur late in the dry austral winter season, during which prior carbonate precipitation was also detected. Since the growth of speleothems is influenced by numerous cave-specific factors, this study, although preliminary, indicates that Anjohibe Cave drip waters are capable of registering changes in its surrounding environment. A longer monitoring study is expected in the future to constrain the timing and the mode of transfer.

Robust reconstructions of paleoclimate and paleoenvironmental changes using stalagmite proxy records critically depend on detailed observations of the transfer function between the regional climate/environment, the karst aquifer hydrology, and finally the cave microclimate via monitoring, which is currently lacking in Madagascar. This paper reports the first monitoring study performed in Anjohibe Cave, in Mahajanga, NW Madagascar to understand the linkage between regional climatological changes and cave responses to such changes. In this research, we monitored (1) the drip water pH, TDS, EC, temperature, δ13CDIC, δ18Ow, δ2Hw, and elemental (Ca, Mg, Sr) composition, and (2) the cave atmosphere pCO2, relative humidity (RH) and temperature. Three significant findings were drawn from the results. First, the data show that air-to-air transfer is fast, and the internal parameters closely vary with the regional climatology. Second, rainfall to drip signal transfer is not immediate, and it can take few months to one season for the signals to be detected in the drip water due to the “epikarst storage effect”. Lastly, CaCO3 precipitation is likely to occur during the winter-summer transition, during which prior carbonate precipitation was detected. Since the growth of speleothems is influenced by numerous cave-specific factors, this study, although preliminary, indicates that Anjohibe Cave drip waters are capable of registering changes in its surrounding environment. Such information is ultimately archived in speleothems to reconstruct paleoclimate and paleoenvironmental changes. Results from this research will be of high significance for those working on speleothems within Madagascar, and for those working on understanding the transfer of climatic variations to cave deposits.

Human activities within the context of a drying trend triggered the megafaunal extinctions in Madagascar and Mascarene Islands., Madagascar and the Mascarene Islands of Mauritius and Rodrigues underwent catastrophic ecological and landscape transformations, which virtually eliminated their entire endemic vertebrate megafauna during the past millennium. These ecosystem changes have been alternately attributed to either human activities, climate change, or both, but parsing their relative importance, particularly in the case of Madagascar, has proven difficult. Here, we present a multimillennial (approximately the past 8000 years) reconstruction of the southwest Indian Ocean hydroclimate variability using speleothems from the island of Rodrigues, located ∼1600 km east of Madagascar. The record shows a recurring pattern of hydroclimate variability characterized by submillennial-scale drying trends, which were punctuated by decadal-to-multidecadal megadroughts, including during the late Holocene. Our data imply that the megafauna of the Mascarenes and Madagascar were resilient, enduring repeated past episodes of severe climate stress, but collapsed when a major increase in human activity occurred in the context of a prominent drying trend.

The climatic controls on the stable carbon isotopic composition (δ13C) of speleothem carbonate are less often discussed in the scientific literature in contrast to the frequently used stable oxygen isotopes. Various local processes influence speleothem δ13C values and confident and detailed interpretations of this proxy are often complex. A better understanding of speleothem δ13C values is critical to improving the amount of information that can be gained from existing and future records. This contribution aims to disentangle the various processes governing speleothem δ13C values and assess their relative importance. Using a large data set of previously published records we examine the spatial imprint of climate-related processes in speleothem δ13C values deposited post-1900 CE, a period during which global temperature and climate data is readily available. Additionally, we investigate the causes for differences in average δ13C values and growth rate under identical climatic conditions by analysing pairs of contemporaneously deposited speleothems from the same caves. This approach allows to focus on carbonate dissolution and fractionation processes during carbonate precipitation, which we evaluate using existing geochemical models. Our analysis of a large global data set of records reveals evidence for a temperature control, likely driven by vegetation and soil processes, on δ13C values in recently deposited speleothems. Moreover, data-model intercomparison shows that calcite precipitation occurring along water flow paths prior to reaching the top of the speleothem can explain the wide δ13C range observed for concurrently deposited samples from the same cave. We demonstrate that using the combined information of contemporaneously growing speleothems is a powerful tool to decipher controls on δ13C values, which facilitates a more detailed discussion of speleothem δ13C values as a proxy for climate conditions and local soil-karst processes.

Madagascar, an island located ~300km off the eastern coast of Africa, is a natural laboratory to study paleoclimate and paleoenvironment. It holds a key position in the Indian Ocean and in Africa, as information from it has particularly helped fill gaps in paleoclimate reconstruction in the Southern Hemisphere, where such information is still scarce. Madagascar is seasonally visited by the Intertropical Convergence Zone (ITCZ) and experiences monsoon during austral summers. Furthermore, it hosts caves where speleothems can be found. Speleothems preserve a range of continuous geochemical records, mainly stable isotopes, that allow scientists to predict changes happening in the past. In Madagascar, speleothem studies have revealed distinct early, mid, and late Holocene climatic regimes that were linked to the latitudinal migration of the ITCZ, and the monsoonal responses associated with the migration. Other speleothem studies revealed evidence of the African Humid Period, rapid climate changes, and most importantly the shift in δ13Cc starting ca. AD 800, that was attributed to anthropogenic activities. Although information from these speleothems is unquestionably significant, there are still gaps in isotopic proxies interpretation, mainly in linking modern environments where these speleothems grew and the signals they preserve. Such modern information is however fundamental to calibrate paleo-based climate and environmental reconstructions in Madagascar, which could be a key to refine their past interpretation. In this study, we performed an in-cave spatial test to understand kinetic isotope effect in Anjohibe Cave and to define oxygen isotopic fractionation between speleothem carbonate and its parent water and carbon isotopic fractionation between speleothem carbonate and the corresponding dissolved inorganic carbon (DIC). Results have been compared with modern calibration studies on speleothems from other locations worldwide, and we found that our data fit within the empirical relationship for cave-specific CaCO3-H2O isotope fractionation, grouping a range of monitored caves worldwide, 1000 ln α = 16.1 (103T-1(°K))–24.6 of Tremaine et al. (2011). Other physico-chemical parameters in Anjohibe Cave have also been measured, and they will be used to discuss potential linkages with the spatial variability in the modern speleothem stable isotopic values and their corresponding parent water and DIC.Tremaine, D.M., Froelich, P.N., Wang, Y., 2011. Speleothem calcite farmed in situ: Modern calibration of δ18O and δ13C paleoclimate proxies in a continuously-monitored natural cave system. Geochim Cosmochim Acta 75, 4929-4950.

A multi-proxy record of past climate from a stalagmite from Bone Cave in northwestern Botswana suggests relatively wet conditions late in the Little Ice Age and progressively drier conditions in the twentieth century. The proxies include surfaces of dissolution, variation in layer-specific width, changing mineralogy, varying abundance of detrital material, varying growth rate, and oxygen and carbon isotope data. They combine to suggest a period of exceptional wetness that can be assigned with greatest certainty to ~1810–1820 CE and may have been wettest in 1817. The same proxy data conversely suggest relatively dry conditions after about 1835, and increasing dryness after about 1930. Bone Cave lies in the summer rainfall zone, at the southern margin of the tropical rain belt associated with the Inter-Tropical Convergence Zone (ITCZ). Previously published data from other records combine with the Bone Cave record to support wetter conditions in the 1700s and until about 1830, and drier conditions thereafter. The period of exceptional wetness in ~1810–1820 recorded in BC97-14 coincides with a decade of exceptional cold in the Northern Hemisphere, and its acme may be coincident with the Tambora eruption and its resultant “year without a summer” in Europe and North America. These results are thus compatible with movement of the austral-summer ITCZ southward (rather than an increase in rain its customary position) when the Northern Hemisphere cooled relative to the Southern Hemisphere. Conversely, they suggest that the ITCZ has not come as far south as frequently during Earth's warming in the past century.

The climatic event between 4.2 and 3.9 ka BP known as the “4.2 ka event” is commonly considered to be a synchronous global drought that happened as one pulse. However, careful comparison of records from around the world shows that synchrony is possible only if the published chronologies of the various records are shifted to the extent allowed by the uncertainties of their age data, that several records suggest a two-pulsed event, and that some records suggest a wet rather than dry event. The radiometric ages constraining those records have uncertainties of several decades if not hundreds of years, and in some records the event is represented by only one or two analyses. This paper reports a new record from Stalagmite DP1 from northeastern Namibia in which high 230Th/232Th activity ratios allow small age uncertainties ranging between only 10–28 years, and the event is documented by more than 35 isotopic analyses and by petrographic observation of a surface of dissolution. The ages from Stalagmite DP1 combine with results from 11 other records from around the world to suggest an event centered at about 4.07 ka BP with bracketing ages of 4.15 to 3.93 ka BP. The isotopic and petrographic results suggest a two-pulsed wet event in northeastern Namibia, which is in the Southern Hemisphere's summer rainfall zone where more rain presumably fell with southward migration of the Inter-Tropical Convergence Zone as the result of cooling in the Northern Hemisphere. Comparison with other records from outside the region of dryness from the Mediterranean to eastern Asia suggests that multiple climatic zones similarly moved southward during the event, in some cases bringing wetter conditions that contradict the notion of global drought.

Although the 8.2 ka event is a well-known globally documented abrupt climate event, its detailed expression in tropical southeastern Africa is poorly constrained. Here we present a high resolution and precisely dated record of the regional hydroclimatic variability between 8.5–7.5 ka BP from a polymorphic speleothem (ABC-1) from northwestern Madagascar. In combination with the exclusive recurrences of calcite layers (indicative of wetter conditions), the distinctly negative excursions in the ABC-1 δ18O record manifest a wet 8.2 ka event that spanned from 8.230 to 8.053 ka BP. The event is characterized by the replacement of shorter-term (interannual to interdecadal) δ18O-δ13C coherence with the longer-term (multidecadal to centennial) coherence, suggesting a stable and well-developed forest system during the 8.2 ka event. Our data reveal a two-stage structure of the 8.2 ka event, which is superimposed with two brief pluvial episodes in each stage. The widely documented two peaks/troughs during Stage-I suggest rapid propagation of climate signal through atmospheric processes triggered initially by the freshwater forcing, whereas the divergent behaviors of the peaks/troughs in Stage-II in different climate systems may suggest a different forcing.

Petrographic features, mineralogy, and stable isotopes from two stalagmites, ANJB-2 and MAJ-5, respectively from Anjohibe and Anjokipoty caves, allow distinction of three intervals of the Holocene in NW Madagascar. The Malagasy early Holocene (between ca. 9.8 and 7.8 ka) and late Holocene (after ca. 1.6 ka) intervals (MEHI and MLHI, respectively) record evidence of stalagmite deposition. The Malagasy middle Holocene interval (MMHI, between ca. 7.8 and 1.6 ka) is marked by a depositional hiatus of ca. 6500 years. Deposition of these stalagmites indicates that the two caves were sufficiently supplied with water to allow stalagmite formation. This suggests that the MEHI and MLHI intervals may have been comparatively wet in NW Madagascar. In contrast, the long-term depositional hiatus during the MMHI implies it was relatively drier than the MEHI and the MLHI. The alternating wet–dry–wet conditions during the Holocene may have been linked to the long-term migrations of the Intertropical Convergence Zone (ITCZ). When the ITCZ's mean position is farther south, NW Madagascar experiences wetter conditions, such as during the MEHI and MLHI, and when it moves north, NW Madagascar climate becomes drier, such as during the MMHI. A similar wet–dry–wet succession during the Holocene has been reported in neighboring locations, such as southeastern Africa. Beyond these three subdivisions, the records also suggest wet conditions around the cold 8.2 ka event, suggesting a causal relationship. However, additional Southern Hemisphere high-resolution data will be needed to confirm this.

The timing and causes of paleoenvironmental changes in Madagascar have been debated, specifically in respect to human activity following the settlement in the late Holocene. Here we present δ18O, δ13C, layer-bounding surfaces, layer-specific width, mineralogy, and distribution of macroholes from Stalagmite MA3 from Anjohibe Cave to provide a detailed understanding of the paleoenvironmental changes in northwestern Madagascar between 370 CE and 1300 CE. The stable isotope records of Stalagmite MA3 are compared with stable isotope records of Stalagmites ANJ94-5 and MA2. Detailed examination of the proxies suggests three distinct intervals of changes. (1) Prior to 795 CE, changes in vegetation seem to have responded to changes in monsoonal rainfall linked to the relative position of the ITCZ. The period between ca. 755 CE and 795 was the driest, and the Stalagmite MA3 record is consistent with sediment records (pollen and lithology) from Lake Mitsinjo, northwestern Madagascar, and with sediment records (fossil pollen and charcoal) from Sainte Luce, southeastern Madagascar. (2) Between 795 CE and 870, the environmental conditions became more favorable, when vegetation recovered from the driest interval. The new conditions must have been suitable for community development in the region as suggested by archaeological evidence around Lake Mitsinjo and the Boeny region, and the establishment of the stone town of Mahilaka. (3) After 870 CE, a gradational change in plant communities from C3 to C4 marks the record until around 1130 CE, after which vegetation was dominated by C4 plants. This change cannot be explained by climate alone, as there is no clear relationship in the climate-sensitive proxies. Instead, it could have been caused by “Tavy”, a variety of “swidden” agriculture practiced in the region.

Two words, Malagasy and Madagascan, have emerged in the literature as English terms to refer to the people, the culture, and other animate and inanimate objects from Madagascar, the choice of which has left room for confusion for users (e.g. Malagasy art or Madagascan art, Malagasy rice or Madagascan rice, Malagasy Republic or Madagascan Republic, and so forth). This article has two aims: (1) understanding the sources of such confusion, and (2) subsequently proposing a unified English term (noun and adjective) that will avoid further misperceptions in written and verbal communications accounting for Madagascar. The results from investigating the Web of Science, from historical documents, and from viewpoints from online survey combine to suggest that the term ‘Malagasy’ must have unintentionally been substituted by the term ‘Madagascan’ from the earliest documented usage until now. These two words have been used interchangeably, and the usage has never been applied consistently. This could have been influenced by the simple assumption that ‘Madagascan’ is an intuitive form, derivative of Madagascar. The word “Malagasy” is not simply a straightforward English–Malagasy translation but there is much meaning to it that relate to the history and the culture of Madagascar. ‘Malagasy’ is unique to Madagascar, and in future usage in the English language, we recommend everyone to use it as a word qualifier for different objects, animals, plants, and the people of Madagascar. Keywords: Malagasy, Madagascan, Madagascar, Terminology misconception

Multiple proxies using variation in δ18O, δ13C, mineralogy, and petrography in a newly generated high-resolution record of Stalagmite DP1 from Dante Cave indicate a linkage between changes in hydroclimate in northeastern Namibia and changes in solar activity and changes in global temperatures. The record suggests that during solar minima and globally cooler conditions (ca. 1660–1710 and ca. 1790–1830), wetter periods (reflecting longer summer seasons) in northeastern Namibia were linked to advances of the Inter-Tropical Convergence Zone (ITCZ) and the Inter-Ocean Convergence Zone (IOCZ) southwestward. A slight southward push of the Angola–Benguela Front (ABF) during such intervals could also be expected, bringing more rainfall inland. On the other hand, drier and warmer periods in northeastern Namibia, inferred from the increasing δ18O trend in Stalagmite DP1 after AD 1715, coincide with globally warmer conditions, and thus a northeastward migration of the ITCZ, specifically with more warming of the Northern Hemisphere (NH). This finding agrees with reducing precipitation observed in the summer rainfall zone of southern Africa since ca. 1900. Therefore, predictions of warming in high-latitude regions of the NH in the next century should suggest that the presently semi-arid climate of northern Namibia may become even drier.

Carbonate bearing materials, such as foraminifera, mollusks shells, or speleothems, have the potential to preserve geochemical and isotopic signatures reflecting the environmental conditions at the time they formed. Beyond the conventional δ18O, information from triple oxygen isotopes (reported as 17Oexcess, which is defined as (ln (δ17O/1000 + 1) − λref ln (δ18O/1000 + 1)) × 106) in these archives promises to be a valuable tool to reconstruct past hydrological processes. The goal of this study is to determine the triple oxygen isotope fractionation between CaCO3 and H2O under well-constrained laboratory conditions. We performed laboratory experiments to precipitate CaCO3 polymorphs, either calcite or aragonite, at temperatures between 10 and 35 °C. We then evaluated the effect of polymorphism, temperature, and solution concentration on the 17Oexcess of CO2 extracted from these carbonates and the 17O isotopic fractionation (17α) between water and CaCO3. The obtained values of 18α and 17α between CO2 extracted from CaCO3 and parent water allow us to calculate the fractionation slope θ (=ln17α/ln18α). Our observations suggest that θ is indistinguishable at temperatures of 10 and 27 °C, but is slightly lower at 35 °C. The lower value at 35 °C may be related to disequilibrium during these experiments. We found that θ is independent of polymorph and of solution concentration, indicating that 17Oexcess is less sensitive than δ18O to these geochemical parameters and can thus be a robust proxy for reconstructing 17Oexcess of parent water.

Horizontal cores from a large stalagmite and two tufa deposits in the entrance to Wonderwerk Cave, South Africa, dated by radiocarbon methods, have provided climate proxy data on late Holocene environments near the cave. The δ18O and δ13C time series from stalagmite Core WW1–3 and tufa Core WW3 correlate well with isotope records for other sites in the summer rainfall zone of southern Africa and suggest that late Holocene warm periods in the Northern Hemisphere, including the Medieval Warm period, Roman Warm period, and Minoan Warm period, were times of increased moisture in this rainfall zone. In contrast, late Holocene cold intervals in the Northern Hemisphere, including the Dark Ages Cold period and Sub-Atlantic Cold period, were times of drier climate in the summer rainfall zone. Comparison of the Wonderwerk records with information on human settlement patterns, agricultural expansion or decline, and population growth or decline, shows that growth occurred preferentially during wetter climate periods and declines, including the abandonment of the important town of Mupungubwe in the Shashe-Limpopo area of northeast South Africa and the fall of Great Zimbabwe, which occurred during periods of low precipitation.

Petrographic features, mineralogy, and stable isotopes from two stalagmites collected from Anjohibe and Anjokipoty Cave allow distinction of three intervals of the Holocene in northwestern Madagascar. The Malagasy early Holocene interval (between ca. 9.8 and 7.8 ka) was wet, and vegetation changes seem to have been controlled by changes in climate. The Malagasy late Holocene interval (after ca. 1.6 ka) also records evidence of wet conditions, but changes in vegetation were influenced by anthropogenic effects, as suggested by the stalagmite δ13C shift. The Malagasy middle Holocene interval seems to be characterized by drier conditions, relative to the early and late Holocene. The alternating wet/dry/wet conditions in northwestern Madagascar during each of these Holocene intervals could be linked to the long-term migration of the Inter-Tropical Convergence Zone (ITCZ). Higher southern hemisphere (SH) insolation and globally colder conditions drove the ITCZ's mean position further south, bringing more rainfall to northwestern Madagascar. This condition was favorable for stalagmite deposition. In contrast, higher northern hemisphere (NH) insolation and globally warmer conditions displaced the ITCZ further north, bringing less rainfall to northwestern Madagascar. This condition was not favorable for stalagmite deposition. The linkage between global cooling and wet conditions in regions of the SH, in response to the southward migration of the ITCZ, is further exemplified at centennial scale by the negative δ18O and δ13C values in northwestern Madagascar during the 8.2 ka cold event when the Atlantic Meridional Overturning Circulation (AMOC) weakened. Weakening of the AMOC led to an enhanced temperature gradient between the two hemispheres, i.e. cold NH and warm SH, shifting the mean position of the ITCZ further south. This brought wet conditions in the SH monsoon regions, such as northwestern Madagascar, and dry conditions in the NH monsoon regions, including the Asian Monsoon and the East Asian Summer Monsoon. This climatic relationship is useful to test for climate models that are used to predict changes in future climate.

Petrographic recognition of layer-bounding surfaces in stalagmites offers an important tool in constructing paleoclimate records. Previous petrographic efforts have examined thickness of layers (a possible proxy for annual rainfall) and alternation of layers in couplets (a possible indicator of seasonality). Layer-bounding surfaces, in contrast, delimit series of layers and represent periods of non-deposition, either because of exceptionally wet or exceptionally dry conditions.Two types of layer-bounding surfaces can be recognized according to explicitly defined petrographic criteria. Type E layer-bounding surfaces are surfaces at which layers have been truncated or eroded at the crest of a stalagmite. Keys to their recognition include irregular termination of layers otherwise present on the stalagmite’s flank, dissolutional cavities, and coatings of non-carbonate detrital materials. Type E surfaces are interpreted to represent wet periods during which drip water became so undersaturated as to dissolve pre-existing stalagmite layers, and thus they necessarily represent hiatuses in the stalagmite record. Type L layer-bounding surfaces are surfaces below which layers become thinner upward and/or layers have lesser lateral extent upward, so that the stalagmite’s layer-specific width decreases. They are thus surfaces of lessened deposition and are interpreted to represent drier conditions in which drip rate slowed so much that little deposition occurred. A Type L surface may, but does not necessarily, represent a hiatus in deposition. However, radiometric age data show that Type L surfaces commonly represent significant hiatuses.These surfaces are significant to paleoclimate research both for their implications regarding climate change (exceptionally wet or dry conditions) and in construction of chronologies in which other data, such as stable isotope ratios, are placed. With regard to climate change, recognition of these surfaces provides paleoclimatological information that can complement or even substitute for geochemical proxies. With regard to chronologies, recognition of layer- bounding surfaces allows correct placement of hiatuses in chronologies and thus correct placement of geochemical data in time series. Attention to changing thickness of annual layers and thus to accumulation rate can also refine a chronology. A chronology constructed with attention to layer-bounding surfaces and to changing layer thickness is much more accurate than a chronology in which hiatuses are not recognized at such surfaces.

The characteristic gullies of central Madagascar–lavakas–vary greatly in abundance over short distances, but existing understanding does not explain why some hillsides should have high concentrations of lavakas when nearby slopes have fewer. We present a GIS analysis of lavaka abundance in relation to bedrock geology and topography, covering two areas in the central highlands: the region near Ambatondrazaka and that around Tsaratanana. Both regions have similar average lavaka density (6 lavakas/km2 in Ambatondrazaka, and 5 lavakas/km2 in Tsaratanana, but local lavaka concentrations vary widely. Individual one-km2 squares can host up to 50 lavakas/km2 in Tsaratanana and up to 150 lavakas/km2 in Ambatondrazaka. We find no predictive relationship between bedrock type and lavaka abundance. There is, however, a relationship between lavakas and slope such that lavakas increase in abundance as slopes get steeper, up to an optimum steepness, beyond which they become less numerous. The optimum steepness for lavaka development is about 10 to 15° in Tsaratanana and 25 to 30° in Ambatondrazaka. Lavakas also seem to favour slopes where the gradient changes locally, with an optimum change in grade somewhere in the range 2 to 5°. Our results provide quantitative constraints on lavaka distribution that can be tested in other areas.

A complete and optimized scheme of lettered marine isotope substages spanning the last 1.0 million years is proposed. Lettered substages for Marine Isotope Stage (MIS) 5 were explicitly defined by Shackleton (1969), but analogous substages before or after MIS 5 have not been coherently defined. Short-term discrete events in the isotopic record were defined in the 1980s and given decimal-style numbers, rather than letters, but unlike substages they were neither intended nor suited to identify contiguous intervals of time. Substages for time outside MIS 5 have been lettered, or in some cases numbered, piecemeal and with conflicting designations. We therefore propose a system of lettered substages that is complete, without missing substages, and optimized to match previous published usage to the maximum extent possible. Our goal is to provide order and unity to a taxonomy and nomenclature that has developed ad hoc and somewhat chaotically over the decades. Our system is defined relative to the LR04 stack of marine benthic oxygen isotope records, and thus it is grounded in a continuous record responsive largely to changes in ice volume that are inherently global. This system is intended specifically for marine oxygen isotope stages, but it has relevance also for oxygen isotope stages recognized in time-series of non-marine oxygen isotope data, and more generally for climatic stages, which are recognized in time-series of non-isotopic as well as isotopic data. The terms “stage” and “substage” in this context are best considered to represent climatostratigraphic units, and thus “climatic stages” and “climatic substages”, because they are recognized from geochemical and sedimentary responses to climate change that may not have been synchronous at global scale.

Tsavorite, the vanadian variety of green grossular, is a high value economic gemstone. It is hosted exclusively in the metasedimentary formations from the Neoproterozoic Metamorphic Mozambique Belt. The deposits are mined in Kenya, Tanzania and Madagascar and other occurrences are located in Pakistan and East Antarctica. They are located within metasomatized graphitic rocks such as graphitic gneiss and calc-silicates, intercalated with meta-evaporites. Tsavorite is found as primary deposits either in nodule (type I) or in quartz vein (type II), and in placers (type III). The primary mineralizations (types I and II) are controlled by lithostratigraphy and/or structure. For the African occurrences, the protoliths of the host-rocks were deposited at the beginning of the Neoproterozoic within a marine coastal sabkha environment, located at the margin of the Congo-Kalahari cratons in the Mozambique Ocean. During the East African-Antarctican Orogeny, the rocks underwent high amphibolite to granulite fades metamorphism and the formation of tsavorite deposits occurred between 650 and 550 Ma. The nodules of tsavorite were formed during prograde metamorphism, calcium coming from sulphates and carbonates, whereas alumina, silicates, vanadium and chromium probably came from clays and chlorite. The veins were formed during the deformation of the metasedimentary platform units which experienced shearing, leading to the formation of fault-filled veins. Metasomatism developed during retrograde metamorphism. The metasedimentary sequences are characterized by the presence of evaporitic minerals such as gypsum and anhydrite, and scapolite. Evaporites are essential as they provide calcium and permit the mobilization of all the chemical elements for tsavorite formation. The H2S-S-8 metamorphic fluids characterized in primary fluid inclusions of tsavorites and the delta B-11 values of coeval dravite confirm the evaporitic origin of the fluids. The V2O3 and Cr2O3 contents of tsavorite range respectively from 0.05 to 7.5 wt.%, while their delta O-18 values are in the range of 9.5-21.1 parts per thousand. The genetic model proposed for tsavorite is metamorphic, based on chemical reactions developed between an initial assemblage composed of gypsum and anhydrite, carbonates and organic matter deposited in a sabkha-like sedimentary basin.

ispartof: RAPID COMMUNICATIONS IN MASS SPECTROMETRY vol:33 issue:7 pages:737-740 ispartof: location:England status: published