Southern California Earthquake Center

Abstract Neodymium (Nd) isotopes ratios are regarded as a key proxy for past ocean circulation despite unsettled debate on marine Nd cycle controls, which opposes a “top‐down” hypothesis (surface sources dominate) and a “bottom‐up” hypothesis (sedimentary sources dominate), each with important implications for Nd isotope proxy interpretation. We suggest a new, basin‐specific framework for interpreting Nd isotope records. Specifically, while Nd isotope records are reliable water‐mass‐mixing proxies in the Atlantic, they are likely better used as proxies of residence time in the Pacific. Here, we test these hypotheses using a computationally efficient, data‐constrained model of the marine Nd cycle. Our results show Atlantic Nd isotopes are dominated by surface sources, while Pacific Nd isotope ratios are strongly influenced by sedimentary sources. We find that shallow, reactive sediments—especially those with extremely radiogenic or unradiogenic isotopic compositions—are crucial to reproduce observed εNd profiles in the Pacific.

IntroductionAerobic methanotrophs and non-methanotrophic methylotrophs drive methane cycling in oxic freshwater lakes. Most knowledge about biological aerobic methane oxidation (MOx) comes from ex situ rate experiments, laboratory cultures, and static measurements of natural abundances.MethodsWe investigated the link between MOx rate constants measured with a novel in situ incubation device and the microbial community in Jordan Lake, a methane-rich freshwater lake in NC, USA. We coupled relative abundances of 16S rRNA genes and quantitative PCR of particulate methane monooxygenase subunit A (pmoA) to methane, oxygen, temperature, and in situ MOx rate constants, all collected using the novel iBag in situ incubation system.ResultsIn 16 incubations spread across 13 months, Methylococcaceae, whose cultured members are obligate aerobic methanotrophs, strongly and inversely correlate with naturally-varying oxygen but not with methane. Non-methanotrophic methylotrophs and facultative aerobic methanotrophs are more abundant (up to 15.4% of amplicons), but do not correlate with either dissolved gas. Methylococcaceae correlate better than all other families in the methane-oxidizing community with the first-order MOx rate constants obtained from the in situ incubation data. Changes in the methane-oxidizing community across incubations were inconsistent between experiments but replicable within parallel incubations. The lack of response of the methanotrophic community to ammonium and organic carbon additions suggest these are not limiting.DiscussionOur results suggest Methylococcaceae primarily drive MOx in Jordan lake, despite often not being the most abundant methanotrophic group, and that high oxygen concentrations may suppress this group independently of their association with lower methane concentrations.

Abstract Highly productive summer phytoplankton blooms in the central North Pacific Subtropical Gyre (NPSG) are an annual occurrence that leads to the export of considerable amounts of surface particulate carbon to depth. The mechanisms that control the formation of these blooms remain unresolved, but iron (Fe) availability may be an important factor. From July to October 2022, a large, persistent phytoplankton bloom was detected near 23.3°N, 154.6°W in satellite imagery and in situ measurements. Elevated Fe concentrations and nitrogen (N 2 ) fixation activity measured within the bloom suggest that high Fe may have supported enhanced diazotrophic activity. To evaluate whether aerosol deposition created favorable conditions for bloom formation, we reconstructed the Fe deposition history of the bloom's source waters by integrating surface water back trajectory analyses with aerosol Fe flux simulations. Our results show that waters that hosted the diatom‐diazotroph assemblage bloom received up to 20% more soluble Fe through aerosol deposition than its surrounding waters, primarily from a strong wet deposition event that occurred approximately 1 month before the bloom. The observed lag between deposition and bloom suggests a delayed biological response to atmospheric Fe inputs. Although this moderate increase does not represent incontrovertible evidence that the bloom was stimulated by aerosol Fe deposition, our findings establish the potential for episodic delivery of atmospheric Fe to stimulate diazotrophic activity and phytoplankton growth over month‐long timescales in the NPSG.

• We generated an enhanced seismic catalog with >6 × more events than regional catalogs. • Our catalog has highly constrained hypocenters and a lower magnitude of completeness. • Seismicity 8 months before mainshock reveals damage localization off the main fault. • The Narlı Fault on which the mainshock nucleated underwent years-long weakening. • The critically stressed Narlı Fault was susceptible to dynamic triggering. Earthquakes are assumed to be unpredictable, but their forecasting may improve if signatures of preparatory processes can be reliably identified through continuous monitoring. Recent results suggest that years to months before large earthquakes, progressive rock weakening can lead to seismicity localization and coalescence, facilitating fault rupture. If this holds generally, comprehensive, years-long analyses of seismicity may help detect transients signaling proximity to large failure. We test this hypothesis by considering the 2023, M W 7.8 Kahramanmaraş, Türkiye, earthquake as a case study. A previous study identified an 8-month long activation of seismicity clusters in a complex fault network within 50 km of the future epicenter. To track earthquake evolution at higher resolution, we developed an enhanced seismic catalog combining deep-learning and classic techniques for the six years preceding the mainshock. Recurrent seismicity on the Narlı Fault, a secondary fault where the mainshock nucleated before propagating onto the major East Anatolian Fault Zone intensified months before the event, exhibiting increased localization and interaction. Moreover, in the weeks preceding the mainshock, seismicity surged on a previously quiescent branch aligning with the future rupture plane, yet no immediate foreshocks were observed in the final hours. We propose that persistent damage-induced weakening near the nucleation region primed the system for failure, ultimately enabling rupture propagation toward the main fault. Our findings underscore the importance of long-term, and consistent high-resolution seismic monitoring and analysis for tracking spatiotemporal seismicity transients that may serve as indicators of proximity to rupture.

Abstract Climate models require boundary condition information, such as vegetation and soil distributions because they influence the mean state climate, and feedbacks can significantly influence regional climate and climate sensitivity to CO 2 forcing. Information about past distributions comes primarily from the paleobotanical record, which is often supplemented by a vegetation model to fill data gaps. For recent past periods such as the Pliocene, a quantitative suitability assessment of these vegetation model simulations is sufficient. However, the Miocene Climate Optimum spanning 16.9–14.7 Ma was the warmest period on Earth over the last ∼25 million years and models struggle to reproduce those conditions for the range of paleogeographies and CO 2 concentrations tested, particularly at high latitudes. Here we bring together the Miocene modeling and data communities to update previous vegetation reconstructions used for climate modeling with a new regional approach that relaxes the requirement for a single model simulation to be used, blending instead simulations forced by different paleogeographies and CO 2 concentrations. This ensures the simulated vegetation is first, and foremost, consistent with the paleorecord and provides a baseline for future comparisons. The reconstruction shows global increases in forest cover at all latitudes as compared to today and extensive C 3 grasslands across the high northern latitudes. Data gaps at high latitudes are filled with vegetation models forced by higher CO 2 concentrations than were required at lower latitudes consistent with the inability of current models to simulate Miocene high latitude warmth.

The critical zone (CZ), extending from the vegetation canopy to unweathered bedrock, hosts coupled hydrologic, geochemical, and biological interactions that regulate soil health, water resources, and ecosystem sustainability. The subsurface CZ can ext

Abstract Coastal upwelling ecosystems associated with strong physical stirring exhibit significant mesoscale hydrographic and biological patchiness. Though many studies have found broad correlations between hydrographic properties (e.g., temperature and salinity) and phytoplankton biomass, we lack a detailed understanding of the mechanisms underlying these correlations. Here, using observational data from coastal waters in the California Current System, we demonstrate that the maximum observed chlorophyll in a water parcel increases with salinity—a conservative water‐mass tracer. This relationship arises from the correlations of vertical salinity and sub‐euphotic zone nitrate profiles. This allows us to define the maximum potential chlorophyll as a function of salinity, and thus nitrate. We show that variations in salinity explain patterns in phytoplankton community structure, and discuss how growth, grazing, and light and micronutrient limitation can generate chlorophyll values below the maximum potential. Our mechanistic explanation provides a novel framework for diagnosing biological patchiness solely through salinity observations.

Abstract Tropical and subtropical hydrological systems are important to water resource management. To improve understanding of these dynamical systems, it is useful to probe their relationship with relevant forcings. Historically, insolation is believed to be a major driver of these dynamics, testable using speleothem stable oxygen isotope () records, whose growth regions and precise chronologies enable detailed investigations of tropical/subtropical hydrology. Here we present a systematic analysis of long speleothem records examining the global relationship between and insolation across different timescales and regions. Our analysis reveals that the relationship between speleothem and insolation varies significantly by region, with no latitude, season, or periodicity of insolation bearing global relevance. We demonstrate that, when comparing speleothem to insolation curves, seasonal influence often cannot be distinguished from physical lags without additional constraints from modeling or theory. Most notably, we identify a previously unrecognized asymmetry whereby coherence in the precessional (19–23 kyr) band frequently collapses during glacial periods while maintaining power in the 100 kyr band. This suggests a fundamental reorganization of the hydrological cycle and its response to orbital forcing during glacial periods. Our results indicate that comparing speleothem to single insolation curves oversimplifies the complex relationship between orbital forcing and hydroclimate variability, highlighting our framework's utility for comprehensively exploring these interactions. These findings advance understanding of how mid‐to‐low latitude hydrology responds to external forcing–knowledge that may prove valuable as we face unprecedented ‐driven climate change.

Abstract Studies of exhumed subduction shear zones indicate that metamorphism and metasomatism of the oceanic lithosphere influence the composition, structure, and rheology of megathrust faults, particularly deep along the plate boundary (>30 km). However, less is known about the effects that fluid‐mediated chemical reactions have on the rheological evolution of oceanic crust at shallower depths, which may control diverse modes of fault slip and down‐stepping of the plate boundary into oceanic crust. Here, we present a structural and geochemical study of fault rocks from the Rodeo Cove thrust zone (RCT) in California to examine feedbacks between deformation and metasomatism of oceanic crust in a shallow subduction thrust environment (<15 km). At the RCT, deformation is accommodated by a dense network of reddish and greenish cataclasites, which surround altered basalt blocks containing abundant calcite veins and cement. Electron microprobe analyses show that the altered basalt is primarily composed of clinopyroxene, albite, chlorite, and pumpellyite, whereas the cataclasite is dominated by ferroaluminoceladonite (K‐ and Fe‐rich mica) and iron‐oxyhydroxides interlayered with well‐crystallized sheets of aluminoceladonite. Our findings suggest that subduction‐related faulting and cataclasis increased permeability within the basalt‐hosted shear zone, promoting extensive K‐metasomatism, first by oxidizing seawater and later by hydrothermal fluids sourced from subducted sediment and/or altered oceanic crust at greater depths. Moreover, contrasting deformation mechanisms between the less altered basalt and strongly K‐metasomatized cataclasite, combined with their constitutive properties quantified from deformation experiments, indicate that K‐metasomatism significantly decreased the frictional strength of oceanic crust causing strain to localize in the RCT.