Scott D. Nodder

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

National Seismic Hazard Model for New Zealand: 2010 Update

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

A model of active faulting in New Zealand

Active fault traces are a surface expression of permanent deformation that accommodates the motion within and between adjacent tectonic plates. We present an updated national-scale model for active faulting in New Zealand, summarize the current understanding of fault kinematics in 15 tectonic domains, and undertake some brief kinematic analysis including comparison of fault slip rates with GPS velocities. The model contains 635 simplified faults with tabulated parameters of their attitude (dip and dip-direction) and kinematics (sense of movement and rake of slip vector), net slip rate and a quality code. Fault density and slip rates are, as expected, highest along the central plate boundary zone, but the model is undoubtedly incomplete, particularly in rapidly eroding mountainous areas and submarine areas with limited data. The active fault data presented are of value to a range of kinematic, active fault and seismic hazard studies.

Microbial control of diatom bloom dynamics in the open ocean

[1] Diatom blooms play a central role in supporting foodwebs and sequestering biogenic carbon to depth. Oceanic conditions set bloom initiation, whereas both environmental and ecological factors determine bloom magnitude and longevity. Our study reveals another fundamental determinant of bloom dynamics. A diatom spring bloom in offshore New Zealand waters was likely terminated by iron limitation, even though diatoms consumed <1/3 of the mixed-layer dissolved iron inventory. Thus, bloom duration and magnitude were primarily set by competition for dissolved iron between microbes and small phytoplankton versus diatoms. Significantly, such a microbial mode of control probably relies both upon out-competing diatoms for iron (i.e., K-strategy), and having high iron requirements (i.e., r-strategy). Such resource competition for iron has implications for carbon biogeochemistry, as, blooming diatoms fixed three-fold more carbon per unit iron than resident non-blooming microbes. Microbial sequestration of iron has major ramifications for determining the biogeochemical imprint of oceanic diatom blooms. Citation: Boyd, P. W., et al. (2012), Microbial control of diatom bloom dynamics in the open ocean, Geophys. Res. Lett., 39, L18601, doi:10.1029/2012GL053448.

Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system

Abstract The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially significant species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km2 with average depth of 615 m. The region is noted for its relatively low levels of phytoplankton biomass and primary production that is iron-limited. In order to evaluate the implications of these attributes for the functioning of this ecosystem a steady-state, 19-compartment model was constructed using Ecopath with Ecosim software of Christensen et al. [ www.ecopath.org ]. The system is driven by primary production that is primarily governed by the supply of iron and light. The total system biomass of 6.28 g C m−2 is very low compared with systems so far modelled with a total system throughput of 1136 g C m−2 year−1. In the model, the Southern Plateau retains 69% of the biomass in the pelagic system and 99% of total production. Although fish are caught demersally, most of their food is part of production in the pelagic system. Top predators represent about 0.3% of total biomass and account for about 0.24 g C m−2 year−1 of food consumed made up of birds 0.058 g C m−2 year−1, seals 0.041 g C m−2 year−1, and toothed 0.094 g C m−2 year−1 and baleen whales 0.051 g C m−2 year−1. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food eaten by non-mesopelagic fish. Mean transfer efficiencies between trophic levels II and IV of 23% are at the high end of the range reported in the literature. In the model, adult fish production is almost completely accounted for by the fisheries take (32%), consumption by seals (7%), toothed whales (21%), other adult fish (13%), and squid (20%). Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively. The gross efficiency of the fishery is 0.018% (catch/primary production). Although not all data come from direct knowledge of this system, the model reflects its general characteristics, namely a low primary production system dominated by the microbial loop, low sedimentation to the seafloor, high transfer efficiencies, a long food web and supporting high-level predators.

Particle transformations and export flux during an in situ iron-stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron-induced increases in phytoplankton production. During the 13-days that the SOIREE bloom was monitored, export fluxes within the iron-fertilised patch were not substantially different to those in waters outside the bloom. On days 11–13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time-lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1–13, however, prohibit prediction of any iron-induced export. This conclusion highlights the difficulties of fully testing the “Iron Hypothesis” and for evaluating the implications for global climate change.

Long‐term slip rates and fault interactions under low contractional strain, Wanganui Basin, New Zealand

The newly mapped Kapiti-Manawatu Fault System (KMFS) in southern North Island, New Zealand, accommodated ∼3.5 km of basement throw over the last 3 Myr. Along-strike throw profiles are generated using seven stratigraphic markers, interpreted from seismic reflection profiles acquired <3 km apart. The profiles are symmetrical about their point of maximum displacement, and cumulative profiles suggest that the reverse fault system behaves coherently. The KMFS originates from the reactivation of extensional structures, with fault lengths remaining constant over time. Contractional deformation started at circa 1750 ± 400 ka. Maximum dip-slip rates along individual faults are 1.77 ± 0.53 and 0.74 ± 0.22 mm yr−1 for the 0–120 and 120–1350 ka periods, respectively. The maximum cumulative throw rates across the KMFS are 4.9 ± 1.5 and 1.5 ± 0.5 mm yr−1 for the same periods. Long-term strain rates across the KMFS are 2–5 times smaller than strain rates in the forearc basin of the Hikurangi subduction margin located less than 100 km to the east. The faults of the KMFS may extend to depth and link with the subducted Pacific plate.

Neotectonics of the offshore Cape Egmont Fault Zone, Taranaki Basin, New Zealand

Abstract The Cape Egmont Fault Zone (CEFZ) is a major structural boundary within the predominantly offshore Taranaki Basin. The northeast‐southwest‐striking principal fault within this zone, the Cape Egmont Fault (CEF), represents the westernmost zone of active deformation associated with the Hikurangi subduction system, and is characterised by normal separation and pronounced surface expression across the Taranaki continental shelf. It has a 53 km long, 1–5 m high seafloor scarp, located 6 km to the east of the Maui‐A production platform, and comprises four segments, each characterised by differences in fault geometry and behaviour. Average slip rates on the CEF for the last 225 000 years range from 0 to 0.8 mm/yr, suggesting concomitant extension rates of 0.1–1.8 mm/yr that are comparable with the deformation rates calculated for onshore active faults in the Taranaki‐Wanganui region. The presence of a seafloor scarp and historic seismicity associated with the CEFZ are considered to be indicative of the ...

Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom

Significance The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for approximately 40% of the global ocean; however, our knowledge of how it is transferred between chemical states and pools is poorly constrained. Here we utilize the isotopic composition of dissolved and particulate iron to fingerprint its transformation in the surface ocean by abiotic and biotic processes. Photochemical and biological reduction and dissolution of particulate iron in the surface ocean appear to be key processes in regulating its supply and bioavailability to marine biota. Iron isotopes offer a new window into our understanding of the internal cycling of Fe, thereby allowing us to follow its biogeochemical transformations in the surface ocean. The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

Pigment fluxes from the Subtropical Convergence region, east of New Zealand: Relationships to planktonic community structure

Abstract A preliminary sediment trap study using photo synthetic pigments as biomarker tracers of pelagic food web processes was conducted in three different water types (subantarctic, Subtropical Convergence, and subtropical), east of New Zealand, in winter and spring 1993. The presence of undegraded pigments in trap samples from water depths of 100–550 m indicate that phytoplankton cells were removed rapidly from surface waters, presumably mainly as sinking intact phytoplankton chains, marine aggregates, or as unconverted pigments in zooplankton waste products. Average pigment budget estimates indicate that <1% day‐1 of chlorophyll a standing stock and <4% of primary production were exported during winter and spring. Microzooplankton grazing (4–92% day‐1 chlorophyll a standing stock and 20–194% primary production) was potentially the most important process affecting particle retention in the upper water column and hence pigment fluxes across all three water types. Bacterial degradation, mesozooplankton ...