Determining the trophic guilds of fishes and macroinvertebrates in a seagrass food web

We established trophic guilds of macroinvertebrate and fish taxa using correspondence analysis and a hierarchical clustering strategy for a seagrass food web in winter in the northeastern Gulf of Mexico. To create the diet matrix, we characterized the trophic linkages of macroinvertebrate and fish taxa present inHalodule wrightii seagrass habitat areas within the St. Marks National Wildlife Refuge (Florida) using binary data, combining dietary links obtained from relevant literature for macroinvertebrates with stomach analysis of common fishes collected during January and February of 1994. Heirarchical average-linkage cluster analysis of the 73 taxa of fishes and macroinvertebrates in the diet matrix yielded 14 clusters with diet similarity ≥ 0.60. We then used correspondence analysis with three factors to jointly plot the coordinates of the consumers (identified by cluster membership) and of the 33 food sources. Correspondence analysis served as a visualization tool for assigning each taxon to one of eight trophic guilds: herbivores, detritivores, suspension feeders, omnivores, molluscivores, meiobenthos consumers, macrobenthos consumers and piscivores. These trophic groups, corss-classified with major taxonomic groups, were further used to develop consumer compartments in a network analysis model of carbon flow in this seagrass ecosystem. The method presented here should greatly improve the development of future network models of food webs by providing an objective procedure for aggregating trophic groups.     

Shell growth and oxygen isotopes in the topshell Osilinus turbinatus: resolving past inshore sea surface temperatures

Shells of the rocky shore intertidal gastropod Osilinus turbinatus (von Born), often abundant in archaeological deposits in the Mediterranean region, are a potential source of data on palaeotemperature, palaeoseasonality and archaeological seasonality. To evaluate this species as a climate archive, investigations of annual patterns of shell growth and of monthly variations in oxygen isotopes in shell carbonates were made on different populations in NW Sicily. Mark-recapture experiments at San Vito lo Capo and Mazzaforno show that O. turbinatus grows almost continuously throughout the year but at different rates in different seasons. Around 75% of the yearly shell growth occurs in the autumn and winter. On average, larger/older shells produce narrower annual growth increments than smaller/younger ones. Conspicuous growth lines in larger/older shells show that growth stops during the hottest part of the summer. Oxygen isotope analyses on monthly collected shells of O. turbinatus from three shores (Cala Grande, Monte Cofano and Mazzaforno) show that the isotope values record temperature variations through the year. In all the datasets, surface seawater temperatures (SSTs) calculated from δ¹⁸O_SHELL mostly underestimate measured SSTs, offsets being generally greater in summer. Minimum annual offsets range from 0.0°C to 0.7°C and maximum annual offsets from 3.1°C to 8.7°C. δ¹⁸O_SHELL values fail to record temperatures higher than 25°C. Careful selection of shells to be analysed can reduce offsets between δ¹⁸O_SHELL temperature estimates and measured SSTs for many parts of the year, except the hottest. Allowing for this, shells of O. turbinatus offer good potential as climate archives and for archaeological studies of seasonal patterns of human foraging for shellfish.     

Textural, isotopic and REE variations in spinel peridotite xenoliths, Massif Central, France

Sr and Nd isotope analyses and REE patterns are presented for a suite of well-documented mantle-derived xenoliths from the French Massif Central. The xenoliths include spinel harzburgites, spinel lherzolites and some pyroxenites. They show a wide range of textures from undeformed protogranular material through porphyroclastic to equigranular and recrystallised secondary types. Textural differences are strongly linked to trace element geochemistry and variations in radiogenic isotope ratios. Many undeformed protogranular xenoliths are Type IA LREE-depleted with MORB-type ε_Sr values between − 30.7 and − 23.6, and ε_Nd values + 13.9 to + 9.4. A second group of undeformed xenoliths are Type IB LREE-enriched with higher ε_Sr values (− 22.7 to − 10.6) and lower ε_Nd values (+ 11.9 to + 5.6). Deformed xenoliths with porphyroclastic, equigranular and secondary recrystallised textures are all Type IB (LREE-enriched, ε_Nd < 6.4, ε_Sr > 11.8). It is proposed that two separate events have given rise to the observed mixing arrays: (1) MORB-source depleted mantle was enriched by a component derived from an enriched mantle. Deformation and recrystallisation accompanied this event. (2) Subsequently, unenriched MORB-source mantle interacted with magmas chemically akin to the host basalts, and enrichment occurred with little deformation. Hypotheses of Tertiary mantle diapirism resulting in isochemical deformation and refinement of protogranular mantle to equigranular mantle are untenable because of differences in REE patterns and isotopic ratios between different textural groups.     

Late glacial and Holocene environmental change in the Lake Baikal region documented by oxygen isotopes from diatom silica

We investigate late glacial and Holocene climate change recorded in Lake Baikal using the oxygen isotope composition of diatom silica (δ¹⁸O_DIAT). Evaporation from the lake is minor, and the temperature fractionations of δ¹⁸O are unable to explain variations in the δ¹⁸O_DIAT record alone. Isotopically, low meltwater input from glaciers may have some influence on δ¹⁸O_DIAT, but the assumed periods of climatic warming and wastage do not coincide with large shifts in δ¹⁸O_DIAT. There is a gradual oxygen isotope lowering from 27.0‰ to 20.6‰ over the late glacial, while, during the Holocene, δ¹⁸O_DIAT values return to relatively high values. Previous studies of the modern oxygen and hydrogen isotope composition of Lake Baikal's inputs reveal that fluvial input to the lake's North Basin are isotopically lower than fluvial input from South Basin rivers. This north–south gradient of river δ¹⁸O and δD is mainly due to the greater input from isotopically low winter precipitation in the north and isotopically higher summer precipitation in the south. As a result, the δ¹⁸O_DIAT record from Lake Baikal can at least in part be explained by varying input from these sources related to seasonal changes in precipitation. Changes in atmospheric conditions may have a role in altering seasonality and the distribution of precipitation over Lake Baikal's catchment. A feedback mechanism is well known linking higher Eurasian spring snow cover extent (ESSC) to the development of anticyclonic conditions and low precipitation the following summer in the areas south of Lake Baikal. A simultaneous increase in the importance of depleted water (snowmelt) input from the north and decreased enriched summer precipitation in the south is needed to explain depletions in δ¹⁸O of lake water and subsequently δ¹⁸O_DIAT during colder periods. The opposite of this situation is required to enrich lake water during warmer periods. The analysis of δ¹⁸O from diatom silica is a useful proxy for environmental change, especially in lakes, like Lake Baikal, where carbonates are absent or diluted. However, analysis must be based on near pure diatom samples as even trace amounts of silt can have a dominating effect on δ¹⁸O_DIAT values.     

An analysis of Apollo lunar soil samples 12070,889, 12030,187, and 12070,891: Basaltic diversity at the Apollo 12 landing site and implications for classification of small‐sized lunar samples

Lunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size <0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.     

The comparative limnology of some New Zealand lakes

The morphometry, environmental conditions, and some physical and chemical data are given for twenty‐four New Zealand lakes: seven are in the southern part of the South Island, the others, excluding Lakes Waikaremoana and Waikareiti, are in the “thermal region” of the central North Island. These lakes form two separate groups and include the largest and deepest lakes in both islands.

The lakes in which stratification was established were found to belong to the class known as warm monomictic Two lakes, Rotorua and Rotoehu, were found to be homothermous throughout the year. Open water surface temperatures did not exceed 23°c in the North Island lakes or 19°c in the South Island, and minimum temperatures recorded were 7°c in the northern lakes and 5°c in the southern. In deep lakes the range of temperature was much less: Wakutipu had a range of 16–8.85°c.

The disappearance level of a Secchi disc varied from 18 m (Lake Taupo) to 0.8 m (Lake Rotongaio). The transparency of the water in the glacial and volcanic lakes was comparable to that of similar lakes in other parts of the world.

It was found that total ions, measured by the ion‐exchange resins reached high concentrations in lakes fed by thermal waters: Rotomahana 14.32m.e./l. By contrast Tikitapu, a seepage lake, had the low concentration of. 0.28m.e./l, and Wakatipu, a glacial fed lake, a concentration of O.58m.e./1. The pH range was 8.1–5.8, the lowest value being in Tikitapu, a volcanic lake. Slightly lower pH values were recorded in the bottom waters of lakes when stratified. Lakes were usually supersaturated with oxygen at the surface, and only a few small lakes showed a hypolimnial deficiency. Silica and phosphate were found in high concentrations in some thermal lakes but nitrogen in the forms measured, was found to be low.     

Modification of the subcontinental mantle beneath East Serbia: Evidence from orthopyroxene-rich xenoliths

Orthopyroxene-rich olivine websterite xenoliths (OWB₂) in Palaeogene basanites in East Serbia are mostly composed of tabular low-Al₂O₃ orthopyroxene (> 70 vol.%, Mg# 85–87) containing tiny Cr spinel inclusions. Orthopyroxene shows a slightly U-shaped primitive mantle-normalized trace element pattern with strong peaks at U and Pb, similar to that of orthopyroxene from normal regional peridotitic mantle. In between the orthopyroxenes are interstitial spaces composed of partially altered olivine (Mg# 85–87), clinopyroxene, Ti-rich spinel, Mg-bearing calcite, K-feldspar, apatite, ilmenite and relicts of a hydrous mineral. Clinopyroxene appears as selvages around orthopyroxene and as coarser euhedral crystals. Trace element patterns of the clinopyroxene selvages resemble those of adjacent orthopyroxene, whereas the coarser ones have flatter and more LREE- and LILE-enriched patterns, similar to that of metasomatic clinopyroxene. The OWB₂ xenoliths are interpreted as having formed in two stages. During Stage I orthopyroxene crystallized, along with some spinel, olivine and probably hydrous phase(s). This original OWB₂ lithology was a hydrous olivine-bearing orthopyroxenite that crystallised from subduction-related SiO₂-saturated, boninite-like magmas. During Stage II the interstitial minerals formed due to infiltration of a low-SiO₂, high-CaO and CO₂-rich external melt, accompanied by decomposition of original H₂O-bearing minerals. The calculated composition of the infiltrating liquid corresponds to a mafic alkaline melt similar to the basanitic host but more enriched in CO₂, LREE and LILE. Metasomatism is interpreted in terms of small degree melts related to the Palaeogene mafic alkaline magmatism.     

Aquatic macrophyte vegetation development in Kråkenes Lake, western Norway, during the late-glacial and early-Holocene

Macrofossil analyses were carried out on the late-glacial and early-Holocene sediments of the radiocarbon-dated master core at Kråkenes Lake, western Norway, to investigate the aquatic vegetation changes. Ranunculus sect. Batrachium and Nitella were the earliest pioneers after deglaciation ca. 12,300 ¹⁴C yr BP. The Allerød vegetational succession was very slow during ca. 1000 ¹⁴C yrs in a cool climate and conditions that were similar to those above tree-line in Norway today. The rapid cooling at the start of the Younger Dryas stadial caused extensive disturbance, and with the development of an active cirque glacier in the catchment, plants and animals were almost exterminated from the lake by inflow of permanently cold and turbid water. Rising temperatures caused the glacier to melt at the end of the Younger Dryas. The biotic response to the rapid warming was immediate, with pioneer Ranunculus sect. Batrachium and Nitella expanding within 1-3 decades, closely followed by other elodeids. The lake witnessed a remarkable isoetid succession, with phases dominated by Limosella aquatica, Subularia aquatica, Elatine hydropiper, Isoetes echinospora, and, later, I. lacustris. About 800 yrs into the Holocene most of the macrophytes declined. The short-lived isoetids became extinct, but other taxa probably survived vegetatively. The reasons for this decline are unknown, but are probably related to nutrient depletion in combination with other factors. About 550 yrs later, I. lacustris and Nymphaea colonised, and a stable flora and vegetation developed. This study illustrates the large and rapid changes that occurred over the first 1400 yrs of the Holocene in the macrophyte flora and vegetation in Kråkenes Lake before stability was attained, pointing to the value of a palaeoecological study in tracing aquatic successions over time, and highlighting our lack of knowledge of the underlying ecological factors responsible for such rapid and marked changes.     

The effects of organic removal treatment on the integrity of δ<Superscript>18</Superscript>O measurements from biogenic silica

Prior to oxygen isotope analysis of biogenic silica by fluorination techniques, sediments must be cleaned of all organic compounds. Despite the expanding volume of research utilising oxygen isotope ratios in biogenic silica in palaeoclimatology, very little is known about the effects of different standard preparation methods for the removal of organic matter on the isotope values. Here we compare a number of methods for the removal of organic matter in order to assess their effectiveness and influence on subsequent isotope measurements. Not all the methods described here effectively removed organic matter, the best treatments were H₂O₂ and ignition in air at 550°C. Ignition in air at 950°C, and heating in vacuum at 250 and 450°C results in marked alteration of isotope values, whereas use of hydrogen peroxide (H₂O₂), nitric acid (HNO₃) and plasma ashing have little or no effect, within the bounds of analytical error. Ignition in air at 550°C was the most effective technique, and induced no alteration of δ¹⁸O_silica, but practical constraints may limit the application of this method routinely. Our findings indicate that considerable caution is required when preparing sediment for isotope analysis of biogenic silica.