ProfessorJamie Howarth

Microbial communities underpin nutrient cycling across aquatic ecosystems and are strongly influenced by human disturbance, yet the implication for functional redundancy remains poorly understood. Lake eutrophication is a global threat to freshwater systems, primarily driven by human activities within lakes and surrounding catchments. We hypothesize that eutrophication alters functional redundancy of key metabolic pathways in lake sediments. To test this, we conducted a national scale metagenomic study of surface sediments from 144 New Zealand lakes spanning a broad nutrient gradient. Increasing eutrophication is associated with reduced taxon-based functional redundancy, the potential of multiple species to perform the same function, indicating a narrower taxa pool supporting core functions. In contrast, abundance-based functional redundancy, characterized by the abundance of organisms that can perform the function, showed pathway-specific responses across trophic gradients. For example, nitrification and denitrification have greater abundance-based functional redundancy under higher nutrient conditions, whereas phosphorus transport shows reduced redundancy. By eroding microbial functional redundancy, eutrophication may increase susceptibility of microbial communities to environmental perturbations, potentially compromising their ability to sustain key metabolic processes. As lakes play key roles in biogeochemical cycles, the lower stability of vital biogeochemical pathways in lake sediments is likely to have an impact across ecosystems globally.

Human colonization has profoundly reshaped the landscapes of Aotearoa New Zealand. Reconstructing the transition from pre-human forests to Māori and later European land use remains challenging because some vegetation components, particularly insect-pollinated trees, shrubs, and aquatic plants, are poorly represented in traditional pollen records. Here, we integrate pollen and sedimentary DNA (sedDNA) analyses from a ∼1400-year sediment record from Lake Waihau, North Island, to reconstruct vegetation dynamics across three major cultural phases: pre-human, Māori, and European. The two proxies provide complementary yet distinct vegetation signals. Pollen primarily reflects regional vegetation, particularly wind-pollinated canopy taxa such as Podocarpaceae and Nothofagaceae, and provides a stronger representation of fern communities, whereas sedDNA captures local catchment vegetation and recovers a greater diversity of broadleaved trees, shrubs, wetlands, and aquatic taxa. Together, the records indicate that pre-human landscape comprised a diverse broadleaved–podocarp forest with relatively even representation of common taxa. During Māori settlement, sedDNA indicates that canopy taxa responded differently to forest clearance and, together with pollen, reveals a heterogeneous landscape of forest remnants, shrublands, fernlands, wetlands, and aquatic habitats rather than a simple fern-dominated landscape. European settlement was associated with the expansion of mānuka/kānuka shrubland, pasture, and exotic plantation species. These findings demonstrate that integrating pollen and sedDNA provides a more complete reconstruction of vegetation change than either proxy alone, highlighting the value of multiproxy approaches for reconstructing ecosystem transformation and establishing ecological baselines for restoration and Anthropocene palaeoecology.

Coseismic landslides are a major driver of landscape evolution and geohazards in active mountain belts, reshaping topography and influencing drainage divide migration. The debris they generate initiates sediment cascades, producing transient changes in sediment flux and river incision. However, accurately quantifying coseismic landslide erosion and deposition remains challenging. Following the 2016 Mw 7.8 Kaikōura earthquake, we developed a 3D method to volumetrically map landslide erosion and debris distribution across hillslopes using a 2 m resolution vertical difference model over 6875 km2. Our results show that large coseismic landslides preferentially eroded rock mass from upper slopes, with most debris retained on-slope. Only 14% of total landslide volume was deposited off slope, with ≥4% within channels. In contrast, traditional 2D connectivity approaches estimated that up to 36% of landslides were channel-connected, with 89% of the total landslide volume available for fluvial transport. This new volumetric mapping demonstrates that the transfer of landslide debris from hillslopes to river systems is primarily governed by post earthquake remobilisation processes rather than immediate delivery following the earthquake.

Remote lacustrine environments in the Southern Hemisphere offer rare opportunities to study past climate variability, unaffected by significant human disturbances. Yet, high-resolution palaeoenvironmental reconstructions for the late Holocene remain rare, limiting the current understanding of ecological responses over the last ∼4000 years, to regional climate dynamics. This study investigates late Holocene climate variability and ecosystem responses inferred from sediment cores of oligotrophic Lake Bright located in central Fiordland, Aotearoa New Zealand. We applied a multi-proxy approach, comprising lipid biomarker analysis (brGDGTs and leaf wax n-alkanes), X-ray fluorescence core scanning (XRF-CS), hyperspectral imaging (HSI), bulk sediment geochemical proxies (biogenic silica, TOC, TN, δ 13C, δ 15N) and compound-specific stable hydrogen (δD) isotope analysis to sediment cores and modern catchment soils, water, and vegetation samples. Our results indicate that over the late Holocene, Lake Bright's productivity was tied to changes in hydroclimate, and in particular to surface runoff dynamics. In this record, phases of enhanced lake productivity correlate with more arid climate intervals, characterised by lower detrital input, independent of temperature fluctuations. In contrast, decreased lake productivity coincides with increased detrital influx, reflecting elevated runoff and enhanced catchment erosion into the lake. We identify a climatic shift between 2500 and 1500 years ago, possibly marking a transition from a centennial-scale stable climate towards more pronounced hydroclimate seasonality and a stronger influence of the Southern Hemisphere Westerly Winds (SHWW) in the region. Our findings demonstrate the local influence of regional atmospheric circulation, modulated by the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) and offers insights for predicting how alpine lake ecosystems might respond to future climate changes.

Physical erosion can impact the carbon cycle over a range of time scales. In mountains, widespread landslide-triggering events can remove significant quantities of soil from landscapes, yet isolating the spatial pattern and processes of erosion is difficult. Here we reconstruct the impacts of a great earthquake (Mw >8.0) on two lake sedimentary systems in New Zealand and use a combination of geochemical proxies to explore organic matter provenance and soil erosion. The two catchments differ notably in their geomorphic properties, with the median slope of the Lake Mapourika catchment of 47° (+11°/−18°) versus 39° (+9°/−13°) in the Lake Paringa catchment. Carbon and nitrogen isotopes, n-alkane abundance, and n-alkane hydrogen isotopes show that in the steeper Lake Mapourika catchment, landslides from the earthquake mobilized aged soil organic matter and rock-derived organic carbon by deep erosion. Rock organic carbon erosion increased from 15% (+14%/−10%) to 24% (+13%/−13%) in the Lake Mapourika catchment following the earthquake. By comparing the slopes, sediment connectivity, and stream power indices, we show that in the steeper Lake Mapourika catchment, erosion mobilized deeper soil and more bedrock than in Lake Paringa throughout the record, implying that bedrock landslides remained a key process both during and after the earthquake. Thus, we find that even along the same fault zone and under similar climatic, geological, and ecological conditions, topography strongly controls physical erosion as well as its response to widespread landsliding events.

Modelling sediment transport is important to understand how fluvial systems respond to climatic change or other transient conditions such as catastrophic sediment release. In natural rivers, heterogeneity of sediment properties and variability of the flow regime result in different modes of transport that all contribute to the total sediment load. Le Minor et al. (2022) presented a sediment transport law for rivers that extends from bed load to suspended load while being relevant for a wide range of grain sizes but not specifically addressing the case of a distribution of grain sizes, which must also consider the interactions between grain classes that are mainly important during the sediment erosion phase. If these interactions are not properly considered, the model overestimates transport rates compared to measured ones. We present a new formalism for the reference shear stress of multiple-size sediments, a parameter governing the onset of transport. We show that using a reference shear stress standardized across datasets improves transport rate predictions made with the model of Le Minor et al. (2022). We show that considering the bed roughness length as a reference transport height for single- and multiple-size sediments significantly improves transport rate predictions. We also suggest that, for multiple-size sediments where the bed surface is not fully mobile, the entrainment coefficient should include a dependency on the fraction of mobile grain sizes at the bed surface, although data are insufficient to add this effect in a definite parameterization. Therefore, a standardized reference shear stress and a transport length adjusted with a common reference height across all sizes appear to be two critical ingredients of a fully functional multi-grain-size total sediment load model based on the disequilibrium length. This adjusted model offers the potential to quantify grain-size-specific sediment fluxes when different modes of transport may be observed simultaneously, paving the way for more informed numerical modelling of fluvial morphodynamics and sediment transfers.

Multi-event paleoseismic records are essential for understanding fault behaviour, characterising seismic hazard, and testing earthquake simulator models. The reverse dextral Alpine Fault is the main Australian-Pacific plate boundary structure through the central and southern South Island of Aotearoa New Zealand. While the paleoseismic record of the southern and central sections is relatively well-understood, in contrast, the northern section of the Alpine Fault (AF-NS), adjacent to the distributed Marlborough Fault System (MFS) remains understudied. We present new on-fault paleoseismic data from Marble Hill, a well-known offset alluvial terrace site next to the Maruia River, including results from three trenches and four pits. Dating of deposits comes from radiocarbon and infra-red stimulated luminescence (IRSL) techniques. IRSL dates indicate that the degradational terrace sequence at Marble Hill spans the last ∼2000 years. Trenches reveal evidence for up to four recent paleoearthquake ruptures with timings of ∼1595–1834 (MRE, most recent event), 952–1406 (E2), 706–933 (E3), and 246–736 CE (E4). Preservation of this record has been possible due to avulsion with ensuing incision and degradation of the Maruia River toward its current north-westerly course. A local AF-NS recurrence interval of 413 ± 103 yr is derived from the three closed paleoearthquake intervals between the MRE and E4. The three youngest events are associated with ∼1.3 m (MRE), and 10.7 ± 1.3 m dextral displacement split between E2 and E3. This is the first multi-event timing and slip record from the AF-NS and our results highlight the inadequacies of earlier work that targeted only the MRE. A longer recurrence interval (than for the central and southern sections) and variable displacement per event at the Marble Hill site are indicative of its location along the Alpine Fault, implying a more complex rupture record along the AF-NS adjacent to the MFS and transitioning toward the Wairau Fault.

Evaluating the influence of earthquakes on erosion, landscape evolution, and sediment-related hazards requires quantifying the number, location, and volume of co-seismic landslides following large earthquakes. Direct measurements of individual landslide volumes are difficult to accomplish and, as a result, rarely attempted. As an alternative, power-law scaling relationships between landslide volume and area—derived from global landslide inventories—are often used to estimate landslide volumes. The accuracy of the volume estimates derived from using these global scaling relationships remain untested and could be a source of significant uncertainty. Here, an ensemble method, adopting a pre- to post-event 2 by 2 m difference model covering an area of 6875 km2, has been developed to accurately estimate the volume of the > 30,000 mapped landslides triggered by the 2016 Mw 7.8 Kaikōura earthquake New Zealand. Our ensemble method estimates the total landslide volume for the Kaikōura earthquake to be 241 (+ 100/ − 69) M m3. This newly quantified total volume estimate of co-seismic landsliding was then compared to the total volumes estimated using globally and locally derived scaling relationships, which were found to underestimate the total landslide volume by up to 52%. As an alternative, we recommend that landslide volumes be estimated by sampling directly from high-quality empirical datasets of individual landslide volumes with similar triggering and physiographic settings.

Abstract Cosmogenic radionuclides (CRNs) have been proposed as a tool for tracking landslide sediment, yet interpreting the in‐situ 10 Be signal through time with respect to the post‐seismic sediment cascade has proved challenging in the few examples where it has been applied. To determine how useful the 10 Be concentration is in providing insights into the fluvial response of co‐seismic sediment delivery, we measured the 10 Be concentration in multiple grain sizes in two catchments in New Zealand affected by the 2016 Kaikōura earthquake. A 6‐year post earthquake time series of 10 Be measurements was compared to a sediment budget produced by differencing lidar surveys over the same study period. Results showed that the 10 Be concentrations in the sand fraction are controlled by the post‐seismic hillslope delivery, whereas the 10 Be concentrations in the coarser grains respond to the total volume of landslide material in the active channel. Faster 10 Be dilution and recovery of the sand fraction indicates that the transport rate of the sediment pulse is highly dependent on grain size. The residence of the fine‐grained sediment is estimated to be within a decade of the earthquake, although it may take several centuries to evacuate the entirety of the landslide pulse. We observe different magnitudes of 10 Be dilution between two study catchments, which is attributable to a difference in connected landslide volume from hillslopes. We also demonstrate that the dilution factor has little relationship to conventional landslide metrics on a global scale. Our analysis suggests that the magnitude of dilution is controlled through a complex relationship between the connected landslide volume, the degree of sediment mixing and the delivery and evacuation time of landslide sediment. Additionally, if coarse sediment is primarily sourced through mass‐wasting events, measuring just the sand 10 Be concentration will bias the catchment‐averaged erosion rate.

Submarine canyons transfer substantial amounts of sediment and organic carbon (OC) into the deep ocean, nourishing deep-sea ecosystems and contributing to the global carbon cycle through OC burial and sequestration. Tracking lateral OC transport through submarine canyon systems is challenged by the deep-ocean setting, difficulties with constraining episodic depositional events, and the need to assess the composition and age of marine and terrestrial organic matter. We apply innovative parallel ramped pyrolysis oxidation-accelerator mass spectrometry and pyrolysis-gas chromatography-mass spectrometry with isotope analyses to track OC age and sources in the 2016 Kaikōura earthquake-triggered, canyon-flushing event that deposited along >1300 km of a submarine canyon-channel system, offshore Aotearoa New Zealand. Specifically, these techniques allow us to determine the ages, sources, and partitioning of OC within the Kaikōura turbidite deposit and test hypotheses of how submarine canyon systems contribute to lateral OC flux and burial. Our results show that, despite considerable canyon floor erosion, substantial amounts of young OC were flushed into the deep sea, with relatively little (∼2 %) pre-Holocene OC contributions. Even without a direct connection between rivers and submarine canyons, most (∼55 %) of the OC in the Kaikōura event bed is from terrestrial sources. However, the deposit also contains substantial amounts (∼22 %) of marine-derived OC and ∼23 % of the material is of unassignable origin. Particle sorting imparts variability on the age and composition of OC within turbidite deposits and along the turbidity current flow path. Terrestrial-derived OC is preferentially older than marine-derived OC and concentrated in coarser particle sizes found more commonly at the deposit base and in proximal settings. Young, marine-derived OC is concentrated at the surface of the deposits and tends to be enriched in finer particle sizes. Such OC partitioning in turbidites supports the relevance of depositional models for predicting and quantifying distribution of OC in deep-sea deposits. Earthquake-triggered, canyon flushing events and resulting turbidites enhance OC burial efficiency and can sequester OC effectively, contributing an important carbon sink to the sedimentary carbon cycle.

Reliable seismic hazard analysis builds upon a robust reconstruction of spatiotemporal rupture variability over multi-millennial timescales. Lacustrine sediment sequences can provide long and complete records of strong seismic shaking due to the generally continuous nature of subaqueous sedimentation, allowing for high record sensitivity, continuity and accurate age control. Seismic shaking, which can originate from different seismic sources in a region, is typically recorded as in-situ deformation structures or deposits resulting from subaqueous mass-wasting, lake water oscillations or earthquake-related processes in the catchment of the lake. Several calibration methods have been developed to relate the type and size of lacustrine sedimentary earthquake imprints to ground shaking parameters for recent events. By integrating multi-lake records with other paleoseismic archives, this quantitative information enabled evaluation of the possible location, extent and magnitude of prehistoric earthquakes. This chapter introduces the principles of quantitative lacustrine paleoseismology and illustrates its value by three comprehensive case studies from intraplate, transform and subduction zone settings.

The freshwater mussel Echyridella spp. (also known as kākahi) is endemic to Aotearoa New Zealand, with 3 different species recorded. Kākahi are critical for maintaining aquatic ecosystem health and have high cultural significance. All 3 species are classified as at risk, and thus the need to better understand their current and historical distribution and abundance across lakes of Aotearoa New Zealand. In this study, we applied a species-specific droplet digital polymerase chain reaction (ddPCR) assay to investigate the historical presence of kākahi DNA preserved along sedimentary sequences retrieved by gravity coring from 9 lakes. Kākahi sedimentary ancient DNA (sedaDNA) from a single species (Echyridella menziesii) was detected within specific layers in 7 of the 9 sediment cores, but it was only consistently identified in 3 lakes, and only in sediment less than ∼500 years old. The kākahi sedaDNA was patchy or absent in the other 6 study lakes, despite the presence of contemporary kākahi populations in at least 2 of them. We assume that no to poor detection resulted from increased distance between coring sites and source populations as well as biotic (e.g., degradation) and abiotic (e.g., sorption) factors specific to each lake affecting sedaDNA preservation in their sediments. Our findings highlight the importance of exercising caution when interpreting results from sedaDNA studies, especially those exploring macroorganisms with low and spatially heterogeneous environmental DNA concentrations.

Recent subduction earthquakes have shown that weakly-coupled sections of the plate interface fault may be susceptible to a range of different interface slip styles, meaning that seismic hazard is difficult to constrain. On the central Hikurangi subduction margin in Aotearoa New Zealand, plate motion over the last 20 years is thought to have been released aseismically via creep and slow slip events, but geologic evidence indicates seismic slip during large earthquakes may have occurred in the past. Due to patchy preservation and large age uncertainties, correlations of coastal coseismic deformation are unable to distinguish between similarly-timed earthquakes on upper plate faults and widespread synchronous rupture on the plate interface. To develop a more spatially and temporally continuous archive of paleoearthquakes, we present a 10,000-year record of submarine turbidites. Using high-resolution radiocarbon dating and age-depth modelling to generate precise core chronologies, we correlate unique episodes of synchronous turbidite triggering within four discrete submarine distributary systems that collectively span ca. 175 km of the central margin. Agreement with the timing of coseismic deformation onshore indicates that turbidites were likely triggered by ground shaking, and that both proxies can be combined to better examine individual earthquake ruptures. For the last 5000-year period, our onshore-offshore correlations show that paleoearthquakes vary in size, location and the combination of source faults involved. Our findings support that seismic slip on the plate interface occurred during some events which may also be part of larger multi-section subduction earthquakes. Based on this, we infer conditional stability on the weakly-coupled central Hikurangi margin that allows the plate interface to host both interseismic slow slip and seismic slip in large earthquakes. Combining the merits of onshore-offshore proxies to investigate variability in paleoearthquake patterns at other subduction margins should be prioritised to help reveal the full scope of interface slip behaviour and associated hazard.

Lake tsunamis generated from subaerial and subaqueous mass movements can pose a significant hazard to lakeshore communities. However, current knowledge of lake tsunamis is derived from a limited number of studies, primarily in low-seismicity settings such as Switzerland and Norway. Underrepresented are studies from active tectonic settings, characterised by high relief, high sediment yields, and frequent seismic shaking that is likely to produce episodic subaerial and subaqueous mass movements with the potential to produce lake tsunamis. This study seeks to address this gap in our understanding through an examination of four lakes in the active tectonic region of New Zealand's South Island (Lake Rotoiti, Lake Rotoroa, Lake Brunner, and Lake Mapourika). High-resolution multibeam bathymetry and high-resolution seismic reflection data were used to identify and characterize the landforms and sediment associations of large and potentially tsunamigenic mass movements. A qualitative evaluation of the tsunamigenic capacity of the identified mass movements was undertaken using established empirical relationships between mass movement volumes and run-up heights. The bathymetric and seismic surveys have revealed 16 previously undocumented mass movements capable of generating lake tsunami. Fluvial deltas and bedrock slopes proximal to active faults were the most prevalent mass movement source areas, with the largest mass movements identified being a 49 Mm3 delta collapse and a 110 Mm3 subaerial mass movement in Lake Rotoroa. The findings suggest the origin and characteristics of the potentially tsunamigenic mass movements identified were strongly influenced by the high rates of delta progradation, frequent seismic activity, and the preconditioning and displacement by active faults, typical of active tectonic settings. Our results provide new insights into the heterogeneity of lacustrine mass movements and the drivers of lake tsunami hazards in underrepresented high-seismicity regions. The findings illustrate the importance of developing regionally specific insights into lake tsunami susceptibility and may inform the direction of more detailed lake tsunami hazard assessment in New Zealand.

A new multi-proxy paleo database for lake ecosystem and catchment change in Aotearoa New Zealand points to the potential resource and ecosystem service roles of Typha orientalis (raupō). In the context of chronic wetland degradation in Aotearoa New Zealand over the past century, this iconic yet enigmatic wetland plant can be viewed, alternately, as an invasive threat; a valuable cultural and economic resource; and a natural, indigenous agent for bioremediation. Our investigation reconstructs the history of raupō over the past ~1000 years, based on 92 new pollen records generated from lake sites across Aotearoa New Zealand. At almost every site where raupō is present today, its expansion is promoted to varying extents during periods of human activity and at 87% of sites investigated, raupō shows its maximum palynological abundance post human arrival. Multiple patterns of response over time point to a range of hydrological, trophic, and cultural scenarios that are conducive for raupō expansion, raising prospects for its potential role in mitigating the ecological impacts of disturbance. Raupō expansion, promoted by anthropogenic forest clearances and associated sediment and nutrient flux, would in turn have provided new opportunities for its use as a valuable food and material resource, prompting further questions as to the extent it was deliberately managed by indigenous populations. As both a benefactor from, and provider for, expanding populations, raupō may be regarded as a human associate in Aotearoa New Zealand prehistory. As well as being indigenous to Aotearoa New Zealand, T . orientalis also occurs naturally in Australia and east Asia and shares the intrinsic ecological and morphological attributes of the ~40 species or hybrids of Typha that span most of the planet. This work therefore may encourage wider application of the genus as a biocultural asset informed from its local natural history.