Year-round Rainfall Research Articles

An Alternative Interpretation for the Origin of Black Shale in the Bakken Formation of the Williston Basin

AbstractRecently, most researchers have argued for a deep-water origin for black shale in the Bakken Formation of the Williston basin based on sedimentology, paleontology and geochemistry characteristics that place black, laminated, sparsely fossiliferous, radiolarian, pyritic shale as the seaward lithology in an assemblage that originally transitioned landward into shallow-water facies. The shallow-water interpretation advocated in this paper is based on stratigraphic characteristics that include the absence of strata that represent a landward equivalent to the shale, gradual landward shale pinchouts, the restriction of Bakken strata to basinal areas, the Bakken onlap depositional style, and placement of a major maximum flooding surface near the base of the overlying Lodgepole.Black mud deposition in the Bakken is interpreted to have occurred in a low-relief, semi-enclosed, epeiric-lagoon environment with typical water depths of 0–30 m. It is theorized that a high-rainfall climate caused salinity stratification that produced bottom-water anoxia, which preserved organic material. Perhumid climate conditions (year-round rainfall) fostered thick soils and dense vegetation that limited sediment release and induced mud-dominated, sand-poor deposition. During middle Bakken deposition, an arid to semi-arid climate eliminated the bottom-water anoxic conditions and caused carbonate-siliciclastic deposition. The abrupt vertical transitions from black shale (lower Bakken), to carbonate-siliciclastic lithologies (middle Bakken), to black shale (upper Bakken) were caused by paleoclimate change.

Classifying and mapping rainfall seasonality in South Africa: a review

ABSTRACTAcross South Africa, many activities warrant consistent and predictable rainfall patterns which can be achieved from classification of rainfall seasonality into homogeneous zones. Since the first seasonality map for South Africa which was published in 1938, attempts to classify and map rainfall seasonality for the region has proliferates, with many different approaches adopted. Currently, 59 published maps depicting rainfall seasonality across South Africa are known. Although the dichotomy between the summer and winter rainfall zones is well-reported, classification and mapping of the seasonal rainfall zones remains somewhat arbitrary and relatively inconsistent amongst authors. Amongst these maps, there is a broad consensus that three distinct regions with a relatively homogenous seasonal distribution of rainfall exist, known as: summer, winter and year-round rainfall zones. Much of the country has been classified with at least 75% spatial agreement, while classification for the remainder of the country remains rather disputed. Available classification approaches were interrogated, and it was demonstrated that each approach is associated with limitations affecting the accuracy of these maps. We suggest a rigorously assessed approach, or a set of approaches should be used to classify rainfall zones which may be important in further understanding rainfall seasonality across South Africa.

Changes in soil carbon stocks after land-use change from native vegetation to pastures in the Atlantic forest region of Brazil

Millions of hectares (ha) of the Atlantic forest of Brazil have been deforested and replaced by pastures, quite a large proportion of this in the last 60 years. There have been few studies on the impact of this land-use change on stocks of soil organic matter (SOM) only one study reported the state of vigour of the pastures. The aim of this study was to estimate the overall change in SOM stocks 16 years after the removal of forest vegetation in this biome in southern Bahia and the installation of pastures of Brachiaria brizantha fertilized with N and maintained under controlled grazing. Soil samples were taken for evaluation of density and texture and for analyses of C and N total and 13C abundance to a depth of 100 cm at 100 m intervals along four transects of 400 m from the pastures into the forest. Grazing was found not to have any significant effect on soil density (compaction). The live weight gain of the Nellore cattle on both cultivars of B. brizantha, fertilized with 120 kg N ha−1 yr−1 as urea during 12 years, was close to 500 kg ha−1 yr−1. The gain in soil C was similar under the two grass cultivars, being approximately 15 Mg C ha−1 to a depth of 30 cm and 20 Mg C ha−1 to 100 cm. The 13C abundance data showed that the large gain in soil C was due to the slow decomposition of the forest-derived C (total loss 12.6 Mg C ha−1 over 16 years) and the large accumulation of C derived from the Brachiaria (total gain 43.2 Mg C ha−1). These results confirm the potential of productive Brachiaria pastures to accumulate soil C in a tropical climate with year-round rainfall.

Diet and feeding in the Cape Cobra, Naja nivea

It is widely reported that African cobras (genus Naja) are generalist predators, but examination of formal literature yields relatively little evidence of this for most species. Here, we review the diet of cape cobras (Naja nivea) based on examination of museum specimens, an extensive literature review, and social media reported natural history observations. We quantify dietary niche breadth and test the hypothesis that cape cobras are generalist predators. Additionally, we test the hypotheses that diet varies significantly (1) across biomes, and (2) across winter, summer and year-round rainfall regions. We gathered 101 feeding records for cape cobras and based on family-level prey frequencies calculated Levins’ measure of niche breadth as B = 6.57, which we standardised to BA = 0.29. We found no association between prey classes and biomes or rainfall region. Our data indicate that cape cobras consume a wide range of vertebrate prey from all four tetrapod classes, including a large number and diversity of snakes. Our findings suggest that cape cobras may be involved in several important ecological processes. Moreover, our study demonstrates the utility and value of collating natural history observations reported on social media platforms.

Sea level and climate change at the southern Cape coast, South Africa, during the past 4.2 kyr

South African coastal lake sediments provide an excellent opportunity to investigate environmental changes such as sea level and climate variability during the Holocene period. In this study we present a sediment record from the coastal lake Groenvlei located in the southern Cape region which is part of South Africa's year-round rainfall zone. In order to improve the understanding of palaeoenvironmental changes in this region, we provide a high-resolution multi-proxy data set derived from geochemical, mineralogical, isotopic and granulometric analyses. The age-depth model is based on 14C and 210Pb dating and reveals a basal age of 4210 +200/−120cal BP. Differences in the mineralogical composition of deposited carbonates reflect changes in the past lake water chemistry, probably caused by variations in both sea level and climate. Compared to the present, mostly drier conditions and a greater marine influence due to a higher sea level are inferred for the period between 4210 and 2710calBP. However, the record also indicates the occurrence of short humid phases during this time, which were probably associated with heavy rainfall events. A transition layer was deposited between 2710 and 1210calBP, probably as a result of reworking of sediment. During this time, the lake passed through a major change finally turning into a freshwater system from at least 1210calBP until the present. Our data indicate that the marine influence on the lake decreased due to a lower sea level and climate became generally more humid after 1210calBP probably resulting in a greater lake-internal and -external bioproductivity. Based on a comparison with other palaeoenvironmental studies from South Africa, our record suggests a prevailing winter rainfall seasonality at the southern Cape coast between 4210 and 2710calBP and a stronger influence of summer rainfall from 1210calBP onwards.

Management of blue gum eucalyptus in California requires region-specific consideration

Blue gum eucalyptus (Eucalyptus globulus) is a large tree native to Australia that was widely planted throughout California for reforestation, building and timber, but in some areas has spread beyond its planted borders and substantially altered wildlands. Due to its fast growth, large size and reproductive potential, blue gum's impacts on native vegetation, wildlife and ecosystem processes are of concern, particularly in areas with reliable year-round rainfall or fog, where it is most likely to spread. Depending on levels of invasion and rate of spread, blue gum may have negative, positive or neutral impacts on fire regimes, water and nutrient availability, understory vegetation and higher trophic levels. Additional research on the abiotic and biotic impacts of blue gum, quantitative estimates of area covered by blue gum, and weed risk assessments that allow for region-specific climatic information and management goals to be incorporated are needed to guide management of blue gum populations.

Vegetation and climate dynamics during the last glacial period in the fynbos-afrotemperate forest ecotone, southern Cape, South Africa

Despite the southern Cape's great climatic and botanical significance (occupying the transition between the temperate and subtropical circulation systems and forming part of a global biodiversity hotspot), palaeoenvironmental data for this region of southern Africa is limited. This study presents pollen, charcoal and sedimentological data preserved in the Vankervelsvlei wetland, situated in the modern year-round rainfall zone at the ecotone between the Fynbos and Afrotemperate Forest biomes. Combining optically stimulated luminescence and radiocarbon dating techniques, it was possible to establish a chronology for a sediment sequence spanning the last 140,000 years, the longest record yet produced in the region. The data suggest that MIS 5d was relatively warmer (low fynbos pollen percentages and Pentzia-type pollen) than later MIS 5, MIS 4 and most of MIS 3 (∼96–37 ka), which were characterised by decreased temperatures (dominance of ericaceous fynbos). The pollen data indicate a complex response to the change from interglacial to glacial conditions, and suggest an important threshold is crossed in regional ecological dynamics. We postulate that during MIS 5d increased summer rainfall under warmer conditions may have offset increased potential evapotranspiration, allowing for the development of more extensive forests. During its early stages of development Vankervelsvlei was more open (increased aquatics and coarse sediment), trapping more longer-distance pollen (Podocarpus). As the mire became more closed, local elements dominated; a succession that is reflected in significant changes in the pollen assemblage, as Podocarpus remains only in trace percentages, but pollen of Canthium and Morella, which occupy nearly identical climatic niches as Podocarpus, increase in abundance. It is suggested that drought stress remains limited during the last glacial period as a result of reduced temperatures, compensating for what may have been a more seasonal winter-dominated rainfall regime, and that changes in the pollen record relate to vegetation succession and the development of the wetland rather than to major changes in moisture availability. Due to the virtual absence of palaeodata from the southern Cape covering MIS 5 to MIS 3, the establishment of this record provides an important contribution to the overall palaeoenvironmental history of the region.

Origin and evolution of fleshy fruit in woody bamboos

Several hypotheses have been suggested to explain the origin of fleshy fruit in monocots. One is that they originated in the understory of tropical regions and another is that fleshy fruit originated in tropical rainforests where high year-round rainfall implies that seasonality is not a limiting factor. Here we identify the time of origin and ecological preferences of woody bamboos to understand the evolution of the fleshy fruit known as the bacoid caryopsis. Bayesian Inference, Maximum Likelihood and molecular dating analyses were run based on eight plastid and two nuclear regions for 68 bamboo species. Climate data and soil parameters were gathered for 464 localities for these species. The ancestral type of caryopsis was reconstructed by parsimony. According to these analyses the bacoid caryopsis may have evolved independently seven times from the Late Miocene to the Early Pliocene and Mid-Pliocene to Mid-Pleistocene via convergent evolution. Our results suggest that in bamboos neither current climatic variables nor soil parameters were significantly correlated with the appearance of this type of fruit, nor do they have a phylogenetic signal. It is remarkable, however, that the first appearance of the bacoid caryopsis in bamboos might be associated with historical preferences for warmer and wetter climate during the Miocene. Further research is needed to identify whether other factors, such as vivipary or dispersal by small animals, rather than climate, could be responsible for the evolution of this trait in woody bamboos.

Ungulate diversity and precipitation history since the Last Glacial Maximum in the Western Cape, South Africa

This study reviews the precipitation history of the winter and year-round rainfall zones in the Western Cape (South Africa) in light of its fossil ungulate communities. Fossil sequences spanning the Last Glacial Maximum (LGM) and Lateglacial through the Holocene document a decline in ungulate richness through time. Based on the observed relationship between ungulate community richness and annual precipitation in Southern and East Africa, this implies increased effective precipitation during the LGM-Lateglacial at sites located in both the winter and year-round rainfall zones. These results are consistent with other lines of paleoenvironmental evidence from the winter rainfall zone, although they contradict records from the year-round rainfall zone that have been interpreted as reflecting aridity. A critical review of these records suggests that the patterns interpreted in terms of aridity can be explained by other mechanisms, including vegetation change. Current evidence is consistent with paleoclimatic models indicating that altered rainfall patterns during the LGM-Lateglacial were primarily related to the position of westerly frontal systems, which were displaced northward due to the expansion of Antarctic sea ice. Seasonal migration of these systems resulted in an expanded winter rainfall zone across much of southwestern Africa, but perhaps with some summer rains reaching the southern coast.

Detecting Freshwater Inputs via Groundwater Discharge to Marina Lagoon, Mediterranean Coast, Egypt

Much work in recent years has reported on the role of submarine groundwater discharge (SGD) on coastal biogeochemistry, but most of those studies have focused on temperate or tropical climates where year-round rainfall recharges surficial aquifers. The aim of this work, however, was to examine SGD behaviors in an arid setting—Marina Lagoon on the Egyptian Mediterranean coast. SGD was estimated via radon surveys and time-series measurements in lagoon waters during two campaigns (wet season in March 2009 and dry season in July 2010). Relatively higher values of radon were detected in March (maximum >30 dpm/L) compared to July (up to 16 dpm/L), which would indicate either enhanced input rates, or lower mixing/atmospheric losses during the wet season. Lower salinity waters within Marina Lagoon were characterized by higher radon and higher concentrations of dissolved inorganic nitrogen (DIN) and silica (DISi), characteristic of groundwater inputs. Based on lagoon and groundwater radon measurements and an advection-diffusion model, SGD average rates between 0.83 to 2.4 × 108 L/day were estimated for both surveys. Since no seasonal pattern was detected, we suspect that either (1) SGD is derived from a regional aquifer not influenced by recharge from local rainfall or (2) local water use for irrigation and domestic purposes artificially recharges the surficial aquifer in the dry summer (tourist) months, which compensates for the lack of rainfall occurring at that time.

Late Quaternary palaeoenvironments of the winter-rainfall zone of southern Africa: Palynological and sedimentological evidence from the Agulhas Plain

South Africa's winter-rainfall zone is a climatically sensitive region, lying at the interface of the earth's temperate and sub-tropical climatic systems. Its seasonally arid climate has generally prevented the preservation of long and organic-rich sedimentary deposits, producing a spatially limited and temporally biased Quaternary record. This paper seeks to address this issue further and develop a fuller understanding of the wider climatic changes in this region during the Late Quaternary. Modern climatic data does not support the definitions of the “winter-rainfall zone” presented in previous syntheses and this factor, amongst others, may explain difficulties in resolving the palaeoenvironmental record in this region. We present palaeoenvironmental evidence from the Agulhas Plain, an area marginal to the modern winter-rainfall zone. Cored sedimentological and palynological records from two pans are integrated with previously reported records of aeolian activity, providing evidence for phases of enhanced aridity and humidity during the Late Quaternary. The record is fragmentary, which may reflect the cycling of sediment between the pans and their accompanying lunette dunes. Lacustrine sedimentary sequences dating to c. 33 ka – > 47 ka provide evidence for enhanced humidity consistent with evidence in the winter and year-round rainfall zones. Increased humidity is also recorded from c. 2 ka, following drier conditions than the present c. 2.7–2.5 ka. Palynological evidence supports the sedimentological interpretations of pan water status and offers rare insights into the nature of the region's unique Fynbos vegetation during the Late Pleistocene.

3,5,5-trimethyl-1-hexanol

Several hypotheses have been suggested to explain the origin of fleshy fruit in monocots. One is that they originated in the understory of tropical regions and another is that fleshy fruit originated in tropical rainforests where high year-round rainfall implies that seasonality is not a limiting factor. Here we identify the time of origin and ecological preferences of woody bamboos to understand the evolution of the fleshy fruit known as the bacoid caryopsis. Bayesian Inference, Maximum Likelihood and molecular dating analyses were run based on eight plastid and two nuclear regions for 68 bamboo species. Climate data and soil parameters were gathered for 464 localities for these species. The ancestral type of caryopsis was reconstructed by parsimony. According to these analyses the bacoid caryopsis may have evolved independently seven times from the Late Miocene to the Early Pliocene and Mid-Pliocene to Mid-Pleistocene via convergent evolution. Our results suggest that in bamboos neither current climatic variables nor soil parameters were significantly correlated with the appearance of this type of fruit, nor do they have a phylogenetic signal. It is remarkable, however, that the first appearance of the bacoid caryopsis in bamboos might be associated with historical preferences for warmer and wetter climate during the Miocene. Further research is needed to identify whether other factors, such as vivipary or dispersal by small animals, rather than climate, could be responsible for the evolution of this trait in woody bamboos.

Native species: a resource for the diversification of forestry production in the lowland humid tropics

Reforestation of abandoned pastures in the lowland per humid tropics by farmers presents conditions distinct from those for which traditional plantation forestry species have been selected, i.e., low intensity site management, compacted soils and high year-round rainfall. The adaptation of 14 exotic (proven plantation species in other tropical regions) and 66 potential native plantation timber species to these conditions was tested in a 6-year-old trial in the lowland humid tropics of Costa Rica. Two exotic species, Gmelina arborea and Acacia mangium, had the highest growth rates, but both were subject to pest attacks that limited the life of the stand. Other exotic species ( Pinus spp. and Eucalyptus spp.) did not express the high growth rates typical of these species in other regions. Native species within the top 20 overall species tended to have higher survival, fewer pests, but lower growth rates than exotics of similar wood qualities. Of the natives tested, only about 10 species had sufficiently high growth rates and good form to be attractive as potential plantation species, these included Vochysia spp., Stryphnodendron excelsum and Terminalia amazonia. Two native species with high growth rates but poor form were Goethalsia meiantha and Sclerolobium guianensis. The native potential plantation species appeared to be well adapted to the low input forestry practiced by farmers in the lowland humid tropics, while the traditional forestry species (exotic to this region) probably require more intensive production systems. If a wide range of production systems is considered, e.g., agroforestry, fuel wood lots, and mixed plantations, the biodiversity of native species provides a resource to potentially fill these different production niches.

Rainfall, food abundance and timing of parturition in African bats.

We used published results from studies in Africa to test the hypothesis that the timing of parturition in the Chiroptera is constrained by rainfall. Comparison of year-round rainfall and insect data at various latitudes showed that insect abundance peaks approximately a month after peak rainfall. A similar comparison of parturition time to rainfall showed that with the possible exception of the molossids, the Microchiroptera commonly give birth a month before peak rainfall. With an average 6-week lactation period in the Microchiroptera, the timing of parturition is such that young bats are weaned just before the period of maximum insect abundance. We suggest that the needs of the young in this post-weaning period may be more important than the energetic demands of lactation on the mother in determining the timing of parturition on an evolutionary scale. A similar conclusion is implied for the African Megachiroptera, but there is insufficient information on their reproduction to adequately test the main hypothesis for these bats.

Holocene evolution of coastal wetlands in wet-tropical northeastern Australia

Pollen in sediments drilled from the Innisfail coastal plain, northeast Queensland, Australia, was examined to reconstruct the evolution of Holocene wetlands in a wet-tropical environment. In contrast to monsoonal Australia, stable environmental conditions created by year-round rainfall and low tidal range caused abrupt, unidirectional transitions in wetland zonation as marine influence changed. This has enabled a detailed reconstruction of the marine transgression and subsequent progradation. Mangroves colonized in response to marine transgression at c. 7400 BP, as riverine mangroves of low salt tolerance migrated up the Mulgrave River. These were replaced around 7000 BP by extensive Rhizophora-dominated mangroves, coinciding with the development of a Rhizophora-dominated community in Wyvuri embayment. Mangroves reached their greatest extent as sea-level rise slowed towards 6000 BP, with a stillstand indicated by a brief return to terrestrial conditions at one site. High freshwater input depressed salt intrusion in the upper reaches of the Mulgrave estuary and prevented the development of hypersalinity in the upper tidal zone. Drainage conditions then controlled whether mangroves were succeeded by freshwater swamp or swamp-forest.

Methods of Rose Clover Establishment into Barmudagrass Sod

Rose clover (Trifolium hirtum All.), a cool-season annual, has been grown on rangelands of California since the late 1940s. 'Overton R18' is the first rose clover cultivar selected for the southeastern USA. Planting method and seeding rate information for rose clover in pasture systems prevalent to this area of year-round rainfall are not available. The objective of this study was to evaluate planting methods and seeding rates for successful overseeding of rose clover into a 'Coastal' bermudagrass [Cynodon dactylon (L.) Pers.] sod. Planting methods evaluated were: (i) undisturbed sod, drill seed, (ii) undisturbed sod, broadcast seed, (iii) drill seed, apply grass desiccant postemergence, and (iv) disk sod 1 to 2 in. deep, broadcast seed, and harrow to cover seed. Seeding rates within each planting method were 4, 8, 12, 16, 20, and 24 Ib/acre. Seedling density was determined approximately 6 wk after planting and dry matter production by two harvests per growing season. Drilling rose clover in an undisturbed or desiccated sod or broadcasting on a shallow disked sod resulted in the best seedling densities and yield. Influence of seeding rate on seedling density and yield was linear (P < 0.01). A seeding rate of 12 Ib/acre was an acceptable compromise between seed cost and forage production. If rose clover is managed for reseeding and first year forage production is not critical, then seeding rates of 6 to 8 Ib/acre are satisfactory.

Warming phases in Southern Africa during the last 150,000 years: an overview

Firmly based palaeoenvironmental data for southern Africa prior to 40,000 yr B.P. are sparse. The only site which is likely to be able to provide useful information for Isotope Stage 6 is the Pretoria Saltpan; warming at the end of this stage appears to have been rapid and to have been associated with increased precipitation in the interior of the sub-continent. In contrast, dry conditions appear to have prevailed in the southwestern areas during the warmest phases of Isotope Stage 5. This regional difference first became apparent during the Pliocene. During the Last Glacial Maximum temperatures based on isotope ratios reached their lowest values of 5–6°C below present, in the southern part of the subcontinent, and widespread dryness prevailed. Warming became quite rapid after 16,000 yr B.P. and was almost everywhere associated with a marked increase in wetness to around (or even above) present levels. This warming phase continued until the beginning of the Holocene; the terminal Pleistocene was, however, marked by desiccation in the arid western areas. Of special interest is the Holocene altithermal, which can be placed between 7000 and 6500 yr B.P. The extent of the temperature rise cannot be specified with any precision, but probably did not exceed the Holocene mean by more than 2°C. In the southern Cape and Karoo more summer rain apparently occurred than during Isotope Stage 2 and in the early Holocene, and this trend towards year-round rainfall appears to have become increasingly pronounced through the later Holocene until about 2000 yr B.P. This argues, in a general way, for an increase in the proportion of summer rainfall with rising temperature in the present winter rainfall areas of southern Africa.

Carbon isotope ratio measurements of succulent plants in southern Africa.

Succulent plants representing 16 families and a variety of growth forms originating from winter, summer, and year-round rainfall regimes in southern Africa were analyzed for carbon isotope ratios. Most families had species with δ13C values indicative of CAM, particularly those from winter and year-round rainfall regimes. Plants with δ13C values intermediate between CAM and C3, indicating flexible photosynthetic pathways, were generally leafy perennials subject to seasonal tissue dehydration. Reproductive tissue tended to have less negative δ13C values than vegetative tissue on the same plant, indicating drought-season origin of the former.