Heat flow in the Western Arctic Ocean (Amerasian Basin)

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 124(8), (2019): 7562-7587, doi: 10.1029/2019JB017587.From 1963 to 1973 the U.S. Geological Survey measured heat flow at 356 sites in the Amerasian Basin (Western Arctic Ocean) from a drifting ice island (T‐3). The resulting measurements, which are unevenly distributed on Alpha‐Mendeleev Ridge and in Canada and Nautilus Basins, greatly expand available heat flow data for the Arctic Ocean. Average T‐3 heat flow is ~54.7 ± 11.3 mW/m2, and Nautilus Basin is the only well‐surveyed area (~13% of data) with significantly higher average heat flow (63.8 mW/m2). Heat flow and bathymetry are not correlated at a large scale, and turbiditic surficial sediments (Canada and Nautilus Basins) have higher heat flow than the sediments that blanket the Alpha‐Mendeleev Ridge. Thermal gradients are mostly near‐linear, implying that conductive heat transport dominates and that near‐seafloor sediments are in thermal equilibrium with overlying bottom waters. Combining the heat flow data with modern seismic imagery suggests that some of the observed heat flow variability may be explained by local changes in lithology or the presence of basement faults that channel circulating seawater. A numerical model that incorporates thermal conductivity variations along a profile from Canada Basin (thick sediment on mostly oceanic crust) to Alpha Ridge (thin sediment over thick magmatic units associated with the High Arctic Large Igneous Province) predicts heat flow slightly lower than that observed on Alpha Ridge. This, along with other observations, implies that circulating fluids modulate conductive heat flow and contribute to high variability in the T‐3 data set.B.V. Marshall of the U.S. Geological Survey (USGS) was critical to the T‐3 heat flow studies and would have been included as a coauthor on this work if he were not deceased. The original T‐3 heat flow data acquisition program was supported by the USGS and by the Naval Arctic Research Laboratory of the Office of Naval Research. Over the decade of USGS research on T‐3 Ice Island, numerous researchers and technical staff, including B.V. Marshall, P. Twichell, D. Scoboria, J. Tailleur, B. Tailleur, and others, spent months on the island and endured difficult and sometimes dangerous conditions to acquire this data set alongside colleagues from other institutions. Outstanding support from the USGS Menlo Park office, transportation and logistics assistance from other U.S. federal government agencies, Arctic expertise supplied by native Alaskan communities, and collaboration with Lamont researchers made this research program possible. B. Lachenbruch and L. Lawver revived interest in this data set in 2016, and they, along with D. Darby and J. K. Hall, provided ancillary information on T‐3 studies. B. Clarke and M. Arsenault assisted with initial data digitization. We thank M. Jakobsson, R. Saltus, and G. Oakey for providing critical shapefiles and other data and R. Jackson and S. Mukasa for clarification on unpublished information. Reviews by J. Hopper, P. Hart, and W. Jokat improved the manuscript, and V. Atnipp Cross and A. Babb were instrumental in completion of data releases. The USGS's Coastal/Marine Hazards and Resources Program supported C.R. and D.H. between 2016 and 2019, and C.R. used office space provided by the Earth Resources Laboratory at the Massachusetts Institute of Technology during completion of this work. Data in Figure 11 were provided by the U.S. Extended Continental Shelf (ECS) Project. The opinions, findings, and conclusions stated herein are those of the authors and the U.S. Geological Survey, but do not necessarily reflect those of the U.S. ECS Project. Any use of trade, firm, or product name is for descriptive purposes only and does not imply endorsement by the U.S. Government. Digital data, metadata, and supporting plots for T‐3 heat flow, navigation, and radiogenic heat content, along with Lamont gravity and magnetics data, are available from Ruppel et al. (2019), and the original T‐3 expedition report with explanatory metadata can be downloaded from Lachenbruch et al. (2019)

Height-related trends in leaf xylem anatomy and shoot hydraulic characteristics in a tall conifer: safety versus efficiency in foliar water transport

• Hydraulic vulnerability of Douglas-fir (Pseudotsuga menziesii) branchlets decreases with height, allowing shoots at greater height to maintain hydraulic conductance (Kshoot) at more negative leaf water potentials (Ψl). • To determine the basis for this trend shoot hydraulic and tracheid anatomical properties of foliage from the tops of Douglas-fir trees were analysed along a height gradient from 5 to 55 m. • Values of Ψl at which Kshoot was substantially reduced, declined with height by 0.012 Mpa m−1. Maximum Kshoot was reduced by 0.082 mmol m−2 MPa−1 s−1 for every 1 m increase in height. Total tracheid lumen area per needle cross-section, hydraulic mean diameter of leaf tracheid lumens, total number of tracheids per needle cross-section and leaf tracheid length decreased with height by 18.4 μm2 m−1, 0.029 μm m−1, 0.42 m−1 and 5.3 μm m−1, respectively. Tracheid thickness-to-span ratio (tw/b)2 increased with height by 1.04 × 10–3 m−1 and pit number per tracheid decreased with height by 0.07 m−1. • Leaf anatomical adjustments that enhanced the ability to cope with vertical gradients of increasing xylem tension were attained at the expense of reduced water transport capacity and efficiency, possibly contributing to height-related decline in growth of Douglas fir.Keywords: growth limitation, hydraulic conductance, water stress, foliar anatomy, Pseudotsuga menziesii, embolismKeywords: growth limitation, hydraulic conductance, water stress, foliar anatomy, Pseudotsuga menziesii, embolis

An Annual Pattern of Native Embolism in Upper Branches of Four Tall Conifer Species

Premise of the study: The Pacific Northwest of North America experiences relatively mild winters and dry summers. For the tall coniferous trees that grow in this region, we predicted that loss in the hydraulic conductivity of uppermost branches would be avoided because of difficulty reversing accumulated emboli in xylem that is always under negative pressure. • Methods: To test this hypothesis, we measured native percent loss in hydraulic conductivity (PLC; the decrease of in situ hydraulic conductivity relative to the maximum) monthly throughout 2009 in branches at the tops (~50 m) of four species in an old growth forest in southern Washington. • Key results: Contrary to our prediction, freeze – thaw cycles resulted in considerable native PLC. Branches showed hydraulic recovery in the spring and after a moderate increase in native embolism that was observed after an unusually hot period in August. The September recovery occurred despite decreases in the leaf and stem water potentials compared to August values. • Conclusions: Recoveries in branches of these trees could not have occurred by raising the water potential enough to dissolve bubbles simply by transporting water from roots and must have occurred either through water absorption through needles and/ or refilling under negative pressure. Excluding the August value, native embolism values correlated strongly with air temperature of the preceding 10 d. For three species, we found that branches with lower wood density had higher specific conductivity, but not greater native PLC than branches with higher wood density, which calls into question whether there is any hydraulic benefit to higher wood density in small branches in those species.Keywords: Tsuga heterophylla, Abies grandis, wood density, hydraulic conductivity, Thuja plicata, Pseudotsuga menziesi

Reduced wood stiffness and strength, and altered stem form, in young antisense 4CL transgenic poplars with reduced lignin contents

• Reduced lignin content in perennial crops has been sought as a means to improve biomass processability for paper and biofuels production, but it is unclear how this could affect wood properties and tree form. • Here, we studied a nontransgenic control and 14 transgenic events containing an antisense 4-coumarate:coenzyme A ligase (4CL) to discern the consequences of lignin reduction in poplar (Populus sp.). During the second year of growth, trees were grown either free-standing in a field trial or affixed to stakes in a glasshouse. • Reductions in lignin of up to 40% gave comparable losses in wood strength and stiffness. This occurred despite the fact that low-lignin trees had a similar wood density and up to three-fold more tension wood. In free-standing and staked trees, the control line had twice the height for a given diameter as did low-lignin trees. Staked trees had twice the height for a given diameter as free-standing trees in the field, but did not differ in wood stiffness. • Variation in tree morphogenesis appears to be governed by lignin · environment interactions mediated by stresses exerted on developing cells. Therefore our results underline the importance of field studies for assessing the performance of transgenic trees with modified wood properties.Keywords: tension wood, lignin, stem form, wood strength, buckling safety factor, transgenic poplar, wood stiffnes

Coordination of leaf structure and gas exchange along a height gradient in a tall conifer

The gravitational component of water potential and frictional resistance during transpiration lead to substantial reductions in leaf water potential (Ψl) near the tops of tall trees, which can influence both leaf growth and physiology. We examined the relationships between morphological features and gas exchange in foliage collected near the tops of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees of different height classes ranging from 5 to 55 m. This sampling allowed us to investigate the effects of tree height on leaf structural characteristics in the absence of potentially confounding factors such as irradiance, temperature, relative humidity and branch length. The use of cut foliage for measurement of intrinsic gas-exchange characteristics allowed identification of height-related trends without the immediate influences of path length and gravity. Stomatal density, needle length, needle width and needle area declined with increasing tree height by 0.70 mm−2 m−1, 0.20 mm m−1, 5.9 × 10−3 mm m−1 and 0.012 mm2 m−1, respectively. Needle thickness and mesophyll thickness increased with tree height by 4.8 × 10−2 mm m−1 and 0.74 μm m−1, respectively. Mesophyll conductance (gm) and CO2 assimilation in ambient [CO2] (Aamb) decreased by 1.1 mmol m−2 s−1 per m and 0.082 μmol m−2 s−1 per m increase in height, respectively. Mean reductions in gm and Aamb of foliage from 5 to 55 m were 47% and 42%, respectively. The observed trend in Aamb was associated with gm and several leaf anatomic characteristics that are likely to be determined by the prevailing vertical tension gradient during foliar development. A linear increase in foliar δ13C values with height (0.042‰ m−1) implied that relative stomatal and mesophyll limitations of photosynthesis in intact shoots increased with height. These data suggest that increasing height leads to both fixed structural constraints on leaf gas exchange and dynamic constraints related to prevailing stomatal behavior.Keywords: leaf anatomy, mesophyll resistance, photosynthesis, growth limitationKeywords: leaf anatomy, mesophyll resistance, photosynthesis, growth limitatio

Safety factors for xylem failure by implosion and air-seeding within roots, trunks and branches of young and old conifer trees

The cohesion-tension theory of water transport states that hydrogen bonds hold water molecules together and that they are pulled through the xylem under tension. This tension could cause transport failure in at least two ways: collapse of the conduit walls (implosion), or rupture of the water column through air-seeding. The objective of this research was to elucidate the functional significance of variations in tracheid anatomical features, earlywood to latewood ratios and wood densities with position in young and old Douglas-fir and ponderosa pine trees in terms of their consequences for the safety factors for tracheid implosion and air-seeding. For both species, wood density increased linearly with percent latewood for root, trunk and branch samples. However, the relationships between anatomy and hydraulic function in trunks differed from those in roots and branches. In roots and branches increased hydraulic efficiency was achieved at the cost of increased vulnerability to air-seeding. Mature wood of trunks had earlywood with wide tracheids that optimized water transport and had a high percentage of latewood that optimized structural support. Juvenile wood had higher resistance to air-seeding and cell wall implosion. The two safety factors followed similar axial trends from roots to terminal branches and were similar for both species studied and between juvenile and mature wood.Keywords: tracheid, cell wall, mature wood, juvenile wood, embolism, water transpor

Murray's law, the ‘Yarrum’ optimum, and the hydraulic architecture of compound leaves

• There are two optima for maximizing hydraulic conductance per vasculature volume in plants. Murray's law (ML) predicts the optimal conduit taper for a fixed change in conduit number across branch ranks. The opposite, the Yarrum optimum (YO), predicts the optimal change in conduit number for a fixed taper. • We derived the solution for YO and then evaluated compliance with both optima within the xylem of compound leaves, where conduits should have a minimal mechanical role. We sampled leaves from temperate ferns, and tropical and temperate angiosperms. • Leaf vasculature exhibited greater agreement with ML than YO. Of the 14 comparisons in 13 species, 12 conformed to ML. The clear tendency towards ML indicates that taper is optimized for a constrained conduit number. Conduit number may be constrained by leaflet number, safety requirements, and the fact that the number of conduits is established before their diameter during development. • Within a leaf, ML compliance requires leaf‐specific conductivity to decrease from petiole to petiolule with the decrease in leaf area supplied. A similar scaling applied across species, indicating lower leaf‐specific petiole conductivity in smaller leaves. Small leaf size should offset lower conductivity, and petiole conductance (conductivity/length) may be independent of leaf size.Keywords: hydraulic efficiency, wood anatomy, network, leaf specific conductivit

Do Ray Cells Provide a Pathway for Radial Water Movement in the Stems of Conifer Trees?

• Premise of the study: The pathway of radial water movement in tree stems presents an unknown with respect to whole-tree hydraulics. Radial profi les have shown substantial axial sap fl ow in deeper layers of sapwood (that may lack direct connection to transpiring leaves), which suggests the existence of a radial pathway for water movement. Rays in tree stems include ray tracheids and/or ray parenchyma cells and may offer such a pathway for radial water transport. This study investigated relationships between radial hydraulic conductivity (k[subscript s-rad]) and ray anatomical and stem morphological characteristics in the stems of three conifer species whose distributions span a natural aridity gradient across the Cascade Mountain range in Oregon, United States. • Methods: The k [subscript s-rad] was measured with a high-pressure fl ow meter. Ray tracheid and ray parenchyma characteristics and water transport properties were visualized using autofl uorescence or confocal microscopy. • Key results: The k[subscript s-rad] did not vary predictably with sapwood depth among species and populations. Dye tracer did not infi ltrate ray tracheids, and infi ltration into ray parenchyma was limited. Regression analyses revealed inconsistent relationships between k[subscript s-rad] and selected anatomical or growth characteristics when ecotypes were analyzed individually and weak relationships between k[subscript s-rad] and these characteristics when data were pooled by tree species. • Conclusions: The lack of signifi cant relationships between k[subscript s-rad] and the ray and stem morphologies we studied, combined with the absence of dye tracer in ray tracheid and limited movement of dye into ray parenchyma suggests that rays may not facilitate radial water transport in the three conifer species studied.Keywords: Ray parenchyma, Hydraulic architecture, Xylem anatomy, Ray tracheids, Drought, Hydraulic conductivity, Conifers, Radial conductivit

Ice-rich (periglacial) vs icy (glacial) depressions in the Argyre region, Mars: a proposed cold-climate dichotomy of landforms

On Mars, so-called “scalloped depressions” are widely observed in Utopia Planitia (UP) and Malea Planum (MP). Typically, they are rimless, metres- to decametres-deep, incised sharply, tiered inwardly, polygonised and sometimes pitted. The depressions seemingly incise terrain that is icy and possibly thermokarstic, i.e. produced by the thermal destabilisation of the icy terrain. Agewise, the depressions are thought to be relatively youthful, originating in the Late Amazonian Epoch.Here, we report the presence of similar depressions in the Argyre region (AR) (30–60° S; 290–355° E). More importantly, we separate and differentiate these landforms into two groups: (ice-rich) periglacial depressions (Type-1); and, (icy) glacial depressions (Type-2a-c). This differentiation is presented to the Mars community for the first time.Based on a suite of morphological and geological characteristics synonymous with ice-complexes in the Lena Peninsula (eastern Russia) and the Tuktoyaktuk Coastlands (Northwest Territories, Canada), we propose that the Type-1 depressions are ice-rich periglacial basins that have undergone volatile depletion largely by sublimation and as the result of thermal destabilisation. In keeping with the terms and associated definitions derived of terrestrial periglacial-geomorphology, ice-rich refers to permanently frozen-ground in which ice lenses or segregation ice (collectively referenced as excess ice) have formed.We suggest that the depressions are the product of a multi-step, cold-climate geochronology:(1) Atmospheric precipitation and surface accumulation of an icy mantle during recent high obliquities.(2) Regional or local triple-point conditions and thaw/evaporation of the mantle, either by exogenic forcing, i.e. obliquity-driven rises of aerial and sub-aerial temperatures, or endogenic forcing, i.e. along Argyre impact-related basement structures.(3) Meltwater migration into the regolith, at least to the full depth of the depressions.(4) Freeze-thaw cycling and the formation of excess ice.(5) Sublimation of the excess ice and depression formation as high obliquity dissipates and near-surface ice becomes unstable.The Type-2 depressions exhibit characteristics suggestive of (supra-glacial) dead-ice basins and snow/ice suncups observed in high-alpine landscapes on Earth, e.g. the Swiss Alps and the Himalayas. Like the Type-1 depressions, the Type-2 depressions could be the work of sublimation; however, the latter differ from the former in that they seem to develop within a glacial-like icy mantle that blankets the surface rather than within an ice-rich and periglacially-revised regolith at/near the surface.Interestingly, the Type-2 depressions overlie the Type-1 depressions at some locations. If the periglacial/glacial morphological and stratigraphical dichotomy of depressions is valid, then this points to recent glaciation at some locations within the AR being precursed by at least one episode of periglaciation. This also suggests that periglaciation has a deeper history in the region than has been thought hitherto. Moreover, if the hypothesised differences amongst the Argyre-based depressions are mirrored in Utopia Planitia and Malea Planum, then perhaps this periglacial-glacial dichotomy and its associated geochronology are as relevant to understanding late period landscape-evolution in these two regions as it is in the AR