Glacial cycles drive variations in the production of oceanic crust

Volcanic evolution of an ultraslow-spreading ridge

Nearly 30% of ocean crust forms at mid-ocean ridges where the spreading rate is less than 20 mm per year. According to the seafloor spreading paradigm, oceanic crust forms along a narrow axial zone and is transported away from the rift valley. However, because quantitative age data of volcanic eruptions are lacking, constructing geological models for the evolution of ultraslow-spreading crust remains a challenge. In this contribution, we use sediment thicknesses acquired from ~4000 km of sub-bottom profiler data combined with ¹⁴C ages from sediment cores to determine the age of the ocean floor of the oblique ultraslow-spreading Mohns Ridge to reveal a systematic pattern of young volcanism outside axial volcanic ridges. Here, we present an age map of the upper lava flows within the rift valley of a mid-ocean ridge and find that nearly half of the rift valley floor has been rejuvenated by volcanic activity during the last 25 Kyr.

Influence of late Pleistocene sea-level variations on midocean ridge spacing in faulting simulations and a global analysis of bathymetry

It is established that changes in sea level influence melt production at midocean ridges, but whether changes in melt production influence the pattern of bathymetry flanking midocean ridges has been debated on both theoretical and empirical grounds. To explore the dynamics that may give rise to a sea-level influence on bathymetry, we simulate abyssal hills using a faulting model with periodic variations in melt supply. For 100-ky melt-supply cycles, model results show that faults initiate during periods of amagmatic spreading at half-rates >2.3 cm/y and for 41-ky melt-supply cycles at half-rates >3.8 cm/y. Analysis of bathymetry across 17 midocean ridge regions shows characteristic wavelengths that closely align with the predictions from the faulting model. At intermediate-spreading ridges (half-rates >2.3 cm/y and [Formula: see text]3.8 cm/y) abyssal hill spacing increases with spreading rate at 0.99 km/(cm/y) or 99 ky (n [Formula: see text] 12; 95% CI, 87 to 110 ky), and at fast-spreading ridges (half-rates >3.8 cm/y) spacing increases at 38 ky (n [Formula: see text] 5; 95% CI, 29 to 47 ky). Including previously published analyses of abyssal-hill spacing gives a more precise alignment with the primary periods of Pleistocene sea-level variability. Furthermore, analysis of bathymetry from fast-spreading ridges shows a highly statistically significant spectral peak (P < 0.01) at the 1/(41-ky) period of Earth's variations in axial tilt. Faulting models and observations both support a linkage between glacially induced sea-level change and the fabric of the sea floor over the late Pleistocene.

The relationship between volcanism and global climate changes in the Tropical Western Pacific over the mid-Pleistocene transition: Evidence from mercury concentration and isotopic composition

Volcanoes are a significant component of the Earth system, influencing the interaction between oceans and the atmosphere over large spatial and temporal scales. Being a volcanically dynamic region, the Tropical Western Pacific (TWP) can significantly impact variations in global climate. However, high-resolution continuous records of volcanic activity in this region are lacking, resulting in significant uncertainties regarding the coupling between the deep earth, climate changes, and atmospheric CO₂ in the TWP. To address this issue, mercury (Hg) levels, isotopic compositions, and Hg/total organic carbon (Hg/TOC) ratios were determined at site U1486 to track volcanic activity throughout the mid-Pleistocene transition (MPT) from 1.3 Myr to 0.6 Myr. Our results of anomalously high Hg concentrations and Hg/TOC ratios provide evidence of time-varying volcanism throughout the MPT. Mercury isotopes in the Hg-enriched sediments were characterized by near-zero Δ¹⁹⁹Hg values, which is consistent with volcanism acting as the primary source of Hg to the sediments. Spectral analysis of the Hg/TOC ratio showed significant periodicity at ~100 kyr and ~ 23 kyr as well as a weaker signal at ~41 kyr consistent with Milankovitch cycles. A cross spectral analysis of Hg/TOC and the LR04 δ¹⁸O stack record suggests that the peak in volcanism lags the temperature minimum by ~6 kyr, and occurs prior to the δ¹⁸O minimum known as the glacial termination by ~14 ± 2 kyr. The records of volcanic activity in this site are also consistent with a prominent rise in atmospheric CO₂ and negative excursion of benthic carbon isotopes throughout the MPT. This study provides direct sedimentary evidence in the TWP of the feedback between volcanic activity, climate change and atmospheric CO₂.

Earth's Complexity Is Non-Computable: The Limits of Scaling Laws, Nonlinearity and Chaos

Current physics commonly qualifies the Earth system as 'complex' because it includes numerous different processes operating over a large range of spatial scales, often modelled as exhibiting non-linear chaotic response dynamics and power scaling laws. This characterization is based on the fundamental assumption that the Earth's complexity could, in principle, be modeled by (surrogated by) a numerical algorithm if enough computing power were granted. Yet, similar numerical algorithms also surrogate different systems having the same processes and dynamics, such as Mars or Jupiter, although being qualitatively different from the Earth system. Here, we argue that understanding the Earth as a complex system requires a consideration of the Gaia hypothesis: the Earth is a complex system because it instantiates life-and therefore an autopoietic, metabolic-repair (M,R) organization-at a planetary scale. This implies that the Earth's complexity has formal equivalence to a self-referential system that inherently is non-algorithmic and, therefore, cannot be surrogated and simulated in a Turing machine. We discuss the consequences of this, with reference to in-silico climate models, tipping points, planetary boundaries, and planetary feedback loops as units of adaptive evolution and selection.

Surface processes forcing on extensional rock melting

Surface processes and magmatism condition the structural evolution of continental rifts and passive margins through mechanical and thermal effects on the lithosphere rheology. However, their inter-relationships in extensional settings are largely unknown. Here, I use coupled thermo-mechanical geodynamic and landscape evolution numerical modeling to assess the links between erosion of rift shoulders, sedimentation within the rift basin and extensional rock melting. Results suggest that, when the crust is thinner than ~40 km, the extension rate is slower than ~2 cm/yr and the mantle potential temperature is below ~1230 °C, efficient surface processes may double crustal melting by Moho lowering and inhibit mantle decompression melting by ~50% through sediment loading within the rift basin. It is thus likely that surface processes significantly influenced the magmatic activity of a number of extensional settings worldwide - e.g. the Mediterranean, the Gulf of California, the Iberia-Newfoundland margin, and the South China Sea. Because magmatism and surface processes affect jointly the geological carbon cycle, the surface processes forcing on extensional rock melting investigated here involves an additional means of linkage between plate tectonics and climate changes.

On Some Properties of the Glacial Isostatic Adjustment Fingerprints

Along with density and mass variations of the oceans driven by global warming, Glacial Isostatic Adjustment (GIA) in response to the last deglaciation still contributes significantly to present-day sea-level change. Indeed, in order to reveal the impacts of climate change, long term observations at tide gauges and recent absolute altimetry data need to be decontaminated from the effects of GIA. This is now accomplished by means of global models constrained by the observed evolution of the paleo-shorelines since the Last Glacial Maximum, which account for the complex interactions between the solid Earth, the cryosphere and the oceans. In the recent literature, past and present-day effects of GIA have been often expressed in terms of fingerprints describing the spatial variations of several geodetic quantities like crustal deformation, the harmonic components of the Earth’s gravity field, relative and absolute sea level. However, since it is driven by the delayed readjustment occurring within the viscous mantle, GIA shall taint the pattern of sea-level variability also during the forthcoming centuries. The shapes of the GIA fingerprints reflect inextricable deformational, gravitational, and rotational interactions occurring within the Earth system. Using up-to-date numerical modeling tools, our purpose is to revisit and to explore some of the physical and geometrical features of the fingerprints, their symmetries and intercorrelations, also illustrating how they stem from the fundamental equation that governs GIA, i.e., the Sea Level Equation.

Coupling between Grand cycles and Events in Earth's climate during the past 115 million years

Geological sediment archives document a rich periodic series of astronomically driven climate, but record also abrupt, severe climatic changes called events, the multi-Myr boundary conditions of which have generally been ascribed to acyclic processes from Earth’s interior dynamics. These events have rarely been considered together within extended time series for potential correlation with long-term (multi-million year, Myr) cycling. Here I show a coupling between events and multi-Myr cycles in a temperature and ice-volume climatic proxy of the geological past 115 Myr. I use Cenozoic through middle Cretaceous climatic variations, as recorded in benthic foraminifera δ¹⁸O, to highlight prominent ~9 and ~36 Myr cyclicities. These cyclicities were previously attributed either to astronomical or tectonic variations. In particular, I point out that most of the well-known events during the past 115 Myr geological interval occur during extremes in the ~9 and ~36 Myr cycling. One exception is the early Cenozoic hyperthermal events including the salient Paleocene-Eocene Thermal Maximum (~56 Ma), which do not match extremes in long-period cyclicities, but to inflection point of these cycles. Specific focus on climatic events, as inferred from δ¹⁸O proxy, suggest that some “events”, marked by gradual trends within the ~9 and ~36 Myr cycle extremes, would principally be paced by long-term cycling, while “events”, recorded as abrupt δ¹⁸O changes nearby cycle extremes, would be rather induced by acyclic processes. The connection between cyclic and acyclic processes, as triggers or feedbacks, is very likely. Such link between cycling and events in Earth’s past climate provides insight into celestial dynamics governing perturbations in Earth’s surface systems, but also the potential connection between external and Earth’s interior processes.

100- kyr cyclicity in volcanic ash emplacement: evidence from a 1.1 Myr tephra record from the NW Pacific

It is a longstanding observation that the frequency of volcanism periodically changes at times of global climate change. The existence of causal links between volcanism and Earth’s climate remains highly controversial, partly because most related studies only cover one glacial cycle. Longer records are available from marine sediment profiles in which the distribution of tephras records frequency changes of explosive arc volcanism with high resolution and time precision. Here we show that tephras of IODP Hole U1437B (northwest Pacific) record a cyclicity of explosive volcanism within the last 1.1 Myr. A spectral analysis of the dataset yields a statistically significant spectral peak at the ~100 kyr period, which dominates the global climate cycles since the Middle Pleistocene. A time-domain analysis of the entire eruption and δ¹⁸O record of benthic foraminifera as climate/sea level proxy shows that volcanism peaks after the glacial maximum and ∼13 ± 2 kyr before the δ¹⁸O minimum right at the glacial/interglacial transition. The correlation is especially good for the last 0.7 Myr. For the period 0.7–1.1 Ma, during the Middle Pleistocene Transition (MPT), the correlation is weaker, since the 100 kyr periodicity in the δ¹⁸O record diminishes, while the tephra record maintains its strong 100 kyr periodicity.

Anomalous K-Pg-aged seafloor attributed to impact-induced mid-ocean ridge magmatism

Eruptive phenomena at all scales, from hydrothermal geysers to flood basalts, can potentially be initiated or modulated by external mechanical perturbations. We present evidence for the triggering of magmatism on a global scale by the Chicxulub meteorite impact at the Cretaceous-Paleogene (K-Pg) boundary, recorded by transiently increased crustal production at mid-ocean ridges. Concentrated positive free-air gravity and coincident seafloor topographic anomalies, associated with seafloor created at fast-spreading rates, suggest volumes of excess magmatism in the range of ~10⁵ to 10⁶ km³. Widespread mobilization of existing mantle melt by post-impact seismic radiation can explain the volume and distribution of the anomalous crust. This massive but short-lived pulse of marine magmatism should be considered alongside the Chicxulub impact and Deccan Traps as a contributor to geochemical anomalies and environmental changes at K-Pg time.

Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis

Between 5 and 6 million years ago, during the so-called Messinian salinity crisis, the Mediterranean basin became a giant salt repository. The possibility of abrupt and kilometre-scale sea-level changes during this extreme event is debated. Messinian evaporites could signify either deep- or shallow-marine deposits, and ubiquitous erosional surfaces could indicate either subaerial or submarine features. Significant and fast reductions in sea level unload the lithosphere, which can increase the production and eruption of magma. Here we calculate variations in surface load associated with the Messinian salinity crisis and compile the available time constraints for pan-Mediterranean magmatism. We show that scenarios involving a kilometre-scale drawdown of sea level imply a phase of net overall lithospheric unloading at a time that appears synchronous with a magmatic pulse from the pan-Mediterranean igneous provinces. We verify the viability of a mechanistic link between unloading and magmatism using numerical modelling of decompression partial mantle melting and dike formation in response to surface load variations. We conclude that the Mediterranean magmatic record provides an independent validation of the controversial kilometre-scale evaporative drawdown and sheds new light on the sensitivity of magmatic systems to the surface forcing.

The first physical evidence of subglacial volcanism under the West Antarctic Ice Sheet

The West Antarctic ice sheet (WAIS) is highly vulnerable to collapsing because of increased ocean and surface temperatures. New evidence from ice core tephra shows that subglacial volcanism can breach the surface of the ice sheet and may pose a great threat to WAIS stability. Micro-CT analyses on englacial ice core tephra along with detailed shard morphology characterization and geochemical analysis suggest that two tephra layers were derived from subglacial to emergent volcanism that erupted through the WAIS. These tephra were erupted though the center of the ice sheet, deposited near WAIS Divide and preserved in the WDC06A ice core. The sources of these tephra layers were likely to be nearby subglacial volcanoes, Mt. Resnik, Mt. Thiel, and/or Mt. Casertz. A widespread increase in ice loss from WAIS could trigger positive feedback by decreasing ice mass and increasing decompression melting under the WAIS, increasing volcanism. Both tephra were erupted during the last glacial period and a widespread increase in subglacial volcanism in the future could have a considerable effect on the stability of the WAIS and resulting sea level rise.

Sea level fall during glaciation stabilized atmospheric CO2 by enhanced volcanic degassing

Paleo-climate records and geodynamic modelling indicate the existence of complex interactions between glacial sea level changes, volcanic degassing and atmospheric CO₂, which may have modulated the climate system's descent into the last ice age. Between ∼85 and 70 kyr ago, during an interval of decreasing axial tilt, the orbital component in global temperature records gradually declined, while atmospheric CO₂, instead of continuing its long-term correlation with Antarctic temperature, remained relatively stable. Here, based on novel global geodynamic models and the joint interpretation of paleo-proxy data as well as biogeochemical simulations, we show that a sea level fall in this interval caused enhanced pressure-release melting in the uppermost mantle, which may have induced a surge in magma and CO₂ fluxes from mid-ocean ridges and oceanic hotspot volcanoes. Our results reveal a hitherto unrecognized negative feedback between glaciation and atmospheric CO₂ predominantly controlled by marine volcanism on multi-millennial timescales of ∼5,000-15,000 years.

The influence of Antarctic subglacial volcanism on the global iron cycle during the Last Glacial Maximum

Marine sediment records suggest that episodes of major atmospheric CO2 drawdown during the last glacial period were linked to iron (Fe) fertilization of subantarctic surface waters. The principal source of this Fe is thought to be dust transported from southern mid-latitude deserts. However, uncertainty exists over contributions to CO2 sequestration from complementary Fe sources, such as the Antarctic ice sheet, due to the difficulty of locating and interrogating suitable archives that have the potential to preserve such information. Here we present petrographic, geochemical and microbial DNA evidence preserved in precisely dated subglacial calcites from close to the East Antarctic Ice-Sheet margin, which together suggest that volcanically-induced drainage of Fe-rich waters during the Last Glacial Maximum could have reached the Southern Ocean. Our results support a significant contribution of Antarctic volcanism to subglacial transport and delivery of nutrients with implications on ocean productivity at peak glacial conditions.

Response to Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply"

Tolstoy reports the existence of a characteristic 100 thousand year (ky) period in the bathymetry of fast-spreading seafloor but does not argue that sea level change is a first-order control on seafloor morphology worldwide. Upon evaluating the overlap between tectonic and Milankovitch periodicities across spreading rates, we reemphasize that fast-spreading ridges are the best potential recorders of a sea level signature in seafloor bathymetry.

Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply"

Olive et al (Reports, 16 October 2015, p. 310) and Goff (Technical Comment, 4 September 2015, p. 1065) raise important concerns with respect to recent findings of Milankovitch cycles in seafloor bathymetry. However, their results inherently support that the Southern East Pacific Rise is the optimum place to look for such signals and, in fact, models match those observations quite closely.

Response to Comment on "Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply"

Huybers et al present new bathymetric spectra from an intermediate-spreading ridge as evidence for a primary contribution of sea level cycles to the morphology of the seafloor. Although we acknowledge the possibility that sea level-modulated magmatic constructions may be superimposed on a first-order tectonic fabric, we emphasize the difficulty of deciphering these different contributions in the frequency domain alone.

Radiocarbon constraints on the extent and evolution of the South Pacific glacial carbon pool

During the last deglaciation, the opposing patterns of atmospheric CO2 and radiocarbon activities (Δ(14)C) suggest the release of (14)C-depleted CO2 from old carbon reservoirs. Although evidences point to the deep Pacific as a major reservoir of this (14)C-depleted carbon, its extent and evolution still need to be constrained. Here we use sediment cores retrieved along a South Pacific transect to reconstruct the spatio-temporal evolution of Δ(14)C over the last 30,000 years. In ∼2,500-3,600 m water depth, we find (14)C-depleted deep waters with a maximum glacial offset to atmospheric (14)C (ΔΔ(14)C=-1,000‰). Using a box model, we test the hypothesis that these low values might have been caused by an interaction of aging and hydrothermal CO2 influx. We observe a rejuvenation of circumpolar deep waters synchronous and potentially contributing to the initial deglacial rise in atmospheric CO2. These findings constrain parts of the glacial carbon pool to the deep South Pacific.

Spontaneous Pattern Formation of Surface Nanodroplets from Competitive Growth

Nanoscale droplets on a substrate are of great interest because of their relevance for droplet-based technologies for light manipulation, lab-on-chip devices, miniaturized reactors, encapsulation, and many others. In this work, we establish a basic principle for symmetrical arrangements of surface nanodroplets during their growth out of oversaturated solution established through solvent exchange, which takes place under simple and controlled flow conditions. In our model system, nanodroplets nucleate at the rim of spherical cap microstructures on a substrate, due to a pulse of oversaturation supplied by a solvent exchange process. We find that, while growing at the rim of the microcap, the nanodroplets self-organize into highly symmetric arrangements, with respect to position, size, and mutual distance. The angle between the neighboring droplets is 4 times the ratio between the base radii of the droplets and the spherical caps. We show and explain how the nanodroplets acquire the symmetrical spatial arrangement during their competitive growth and why and how the competition enhances the overall growth rate of the nucleated nanodroplets. This mechanism behind the nanodroplet self-organization promises a simple approach to control the location of droplets with a volume down to attoliters.

Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply

Recent studies have proposed that the bathymetric fabric of the seafloor formed at mid-ocean ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth.

Response to Comment on "Glacial cycles drive variations in the production of oceanic crust"

Goff comments that faulting is important for creation of abyssal hills and is the dominant process at slow-spreading ridges. We respond that faulting is indeed important but cannot alone explain the bathymetric signal predicted by our models and observed at the Australian-Antarctic Ridge. We show that for intermediate- to fast-spreading ridges, abyssal hill spacing is consistent with the periodicity of the obliquity cycle.