Description: Many critical processes of ecosystem function, including trophic relationships between predators and prey and maximum rates of photosynthesis and growth, are size-dependent. Size spectral data are therefore precious to modellers because they can constrain model predictions of size-dependent processes. Here we illustrate a multi-step statistical approach to create size spectra based on a reanalysis of plankton size data from the IronEx II experiment, where iron was added to a marked patch of water and changes in productivity and community structure were followed. First, bootstrapping was applied to resample original size measurements and cell counts. Kernel density estimation was then used to provide nonparametric descriptions of density versus size. Finally, parametric distributions were used to obtain parameter estimates that can more easily be applied in models. A major advantage of this approach is that it provides confidence envelopes for the density distributions. Our analyses suggest three basic distributional patterns of cell concentration versus logarithm of equivalent spherical diameter for individual taxa. Composite size-densities of heterotrophs and photoautotrophs reveal important aspects of the coupling between protist grazing and the phytoplankton community.

Description: Episodes of ultrahigh-temperature (UHT, ≥900 °C) granulite metamorphism have been recorded in mountain belts since the Neoarchean. However, evidence for the tectonic mechanisms responsible for the generation of such extreme thermal conditions is rarely preserved. Here we report the discovery of 16 Ma UHT granulites—the youngest identified at the Earth’s surface—from the Kobipoto Mountains of Seram in eastern Indonesia. UHT conditions were produced by a modern tectonic system in which slab rollback–driven lithospheric extension caused core complex–style exhumation of hot subcontinental lithospheric mantle. Overlying continental crust, heated and metamorphosed by exhumed lherzolites, developed spinel + quartz and sapphirine-bearing residual assemblages, shown by phase equilibria modeling to have required temperatures of ~950 °C at ~8 kbar pressure. Seram is therefore a possible modern analogue for ancient orogens that incorporate UHT granulites.

Description: The timing of widespread continental emergence is generally considered to have had a dramatic effect on the hydrological cycle, atmospheric conditions, and climate. New secondary ion mass spectrometry (SIMS) oxygen and laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle Complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton. A 2535 ± 7 Ma monzogranite is characterized by magmatic zircon with slightly elevated 18 O (6.0–7.5 relative to Vienna standard mean ocean water [VSMOW]), consistent with some contribution to the magma from reworked supracrustal material. A supracrustal contribution to magma genesis is supported by the presence of metasedimentary rock enclaves, a large population of inherited zircon grains, and subchondritic zircon Hf ( Hf = –6.6 to –4.1). A separate, distinct crustal source to the same magma is indicated by inherited zircon grains that are dominated by low 18 O values (2.5–4.8, n = 9 of 15) across a range of ages (3536–2598 Ma; Hf = –18.2 to +0.4). The low 18 O grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low 18 O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low 18 O source. Both scenarios require shallow crustal magmatism in emergent crust, to allow interaction with rocks altered by hydrothermal meteoric water in order to generate the low 18 O zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low 18 O magma with which residual detrital zircons were able to exchange oxygen, while preserving their U-Pb systematics. In the second scenario, wholesale melting of the altered rocks occurred in several distinct events through the Mesoarchean, generating low 18 O magma from which zircon crystallized. Ultimately, in either scenario, the low 18 O zircons were entrained as inherited grains in a Neoarchean granite. The data suggest operation of a modern hydrological cycle by the Neoarchean and add to evidence for the increased emergence of continents by this time.

Description: Granitoids with juvenile signatures are common in arc environments and contribute to growth of the continental crust. Thermo-mechanical models of arcs suggest that intermittent intrusion of magma batches leads to magma hybridization, remelting, and remobilization of earlier intrusive rocks driven by fluctuations in temperature and water fluxing. While there are numerous examples in the literature of multiple intrusions and magma hybridization, field examples of remelting and remobilization of earlier intrusive rocks within an arc are rare. Here, we investigate the evolution of magmatic rocks of the Paleoproterozoic St. Peter Suite, emplaced along the SW margin of the Gawler craton, South Australia, a typical calc-alkaline arc suite. Magmatic rocks recording multiple intrusions and multiple magma interactions have undergone in situ remelting and remobilization forming migmatites. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon dating yielded crystallization ages of 1647 ± 12 Ma for a tonalitic gneiss representing the oldest intrusive suite, and 1604 ± 12 Ma for a leucogranite representing the youngest intrusive suite. Both these suites developed synmagmatic foliation and magmatic banding defined by broadly parallel dikes, elongated enclaves, and schlieren. The rock suites record two deformation events associated with anatexis. The first event, D 1 , was responsible for a dominant approximately E-W–striking foliation (S 1 ) parallel to magmatic foliation, and associated with a dominantly sinistral shearing that affected the older suite. This deformation was associated with the first anatectic event, as demonstrated by the association between leucosomes and structures. The second deformation event, D 2 , affected both suites and was characterized by isolated F 2 folds and shear planes filled with leucosomes subparallel to axial planar foliation. Leucosomes interconnected and gave rise to magma extraction channels tens of meters long. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb titanite dating of a late leucosome sample, collected from within the older magmatic suite, yielded an age of 1605 ± 7 Ma, coinciding with the crystallization age of the younger suite rather than postdating it, as expected. We interpret these results to indicate that crystallization of the younger suite and the second anatectic event occurred in the time encompassed by the error of these young ages. Leucosomes from both anatectic phases lack anhydrous peritectic phases and are interpreted to represent low-temperature anatexis resulting from water fluxing. Combined, these results suggest that a protracted and complex intrusive history can be made significantly more complex by anatexis, giving rise to evolved magmas after older ones, erasing earlier intrusive relationships, and establishing new ones. Rocks of the St. Peter Suite record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger and more fractionated magmas.