More Like this

1. The upper ocean silicon cycle of the subarctic Pacific during the EXPORTS field campaign

   Brzezinski, Mark A. ; Varela, Diana E. ( February 2022 , Elementa)

   The contribution of diatoms to the production and export of organic carbon is highly modified in high-nutrient low-chlorophyll (HNLC) regions due to the decoupling of upper-ocean silicon and carbon cycling caused by low iron. The Si cycle and the role of diatoms in the biological carbon pump was examined at Ocean Station Papa (OSP) in the HNLC region of the northeastern subarctic Pacific during the NASA EX port Processes in the Ocean from RemoTe Sensing (EXPORTS) field study. Sampling occurred during the annual minimum in surface silicic acid concentration, [Si(OH)4 ]. Biogenic silica (bSi) concentrations were low being in the tens of nanomolar range despite high [Si(OH) 4 ], ~15 μM. On average the > 5.0 μm particle size fraction dominated Si dynamics accounting for 65% of bSi stocks and 81% of Si uptake compared to the small fraction (0.6 - 5.0 μm). Limitation of Si uptake was detected in the small, but not the large, size fraction. Small diatoms were co-limited with growth rate restricted by Fe and Si uptake restricted by [Si(OH) 4 ], whereas larger diatoms were only growth limited by Fe. About a third of silica production was exported out of the upper 100 m. Themore »contribution of diatoms to carbon export (9 - 13%) was about twice their contribution to primary productivity (3 - 7%). The combination of low silica production, low diatom primary productivity and high bSi export efficiency at OSP was more similar to the dynamics in the subtropical gyres than to other HNLC regions.« less
2. Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

   https://doi.org/10.1126/science.abh3767  

   Mason, Rachel E. ; Craine, Joseph M. ; Lany, Nina K. ; Jonard, Mathieu ; Ollinger, Scott V. ; Groffman, Peter M. ; Fulweiler, Robinson W. ; Angerer, Jay ; Read, Quentin D. ; Reich, Peter B. ; et al ( April 2022 , Science)

   BACKGROUND The availability of nitrogen (N) to plants and microbes has a major influence on the structure and function of ecosystems. Because N is an essential component of plant proteins, low N availability constrains the growth of plants and herbivores. To increase N availability, humans apply large amounts of fertilizer to agricultural systems. Losses from these systems, combined with atmospheric deposition of fossil fuel combustion products, introduce copious quantities of reactive N into ecosystems. The negative consequences of these anthropogenic N inputs—such as ecosystem eutrophication and reductions in terrestrial and aquatic biodiversity—are well documented. Yet although N availability is increasing in many locations, reactive N inputs are not evenly distributed globally. Furthermore, experiments and theory also suggest that global change factors such as elevated atmospheric CO 2 , rising temperatures, and altered precipitation and disturbance regimes can reduce the availability of N to plants and microbes in many terrestrial ecosystems. This can occur through increases in biotic demand for N or reductions in its supply to organisms. Reductions in N availability can be observed via several metrics, including lowered nitrogen concentrations ([N]) and isotope ratios (δ 15 N) in plant tissue, reduced rates of N mineralization, and reduced terrestrial Nmore »export to aquatic systems. However, a comprehensive synthesis of N availability metrics, outside of experimental settings and capable of revealing large-scale trends, has not yet been carried out. ADVANCES A growing body of observations confirms that N availability is declining in many nonagricultural ecosystems worldwide. Studies have demonstrated declining wood δ 15 N in forests across the continental US, declining foliar [N] in European forests, declining foliar [N] and δ 15 N in North American grasslands, and declining [N] in pollen from the US and southern Canada. This evidence is consistent with observed global-scale declines in foliar δ 15 N and [N] since 1980. Long-term monitoring of soil-based N availability indicators in unmanipulated systems is rare. However, forest studies in the northeast US have demonstrated decades-long decreases in soil N cycling and N exports to air and water, even in the face of elevated atmospheric N deposition. Collectively, these studies suggest a sustained decline in N availability across a range of terrestrial ecosystems, dating at least as far back as the early 20th century. Elevated atmospheric CO 2 levels are likely a main driver of declines in N availability. Terrestrial plants are now uniformly exposed to ~50% more of this essential resource than they were just 150 years ago, and experimentally exposing plants to elevated CO 2 often reduces foliar [N] as well as plant-available soil N. In addition, globally-rising temperatures may raise soil N supply in some systems but may also increase N losses and lead to lower foliar [N]. Changes in other ecosystem drivers—such as local climate patterns, N deposition rates, and disturbance regimes—individually affect smaller areas but may have important cumulative effects on global N availability. OUTLOOK Given the importance of N to ecosystem functioning, a decline in available N is likely to have far-reaching consequences. Reduced N availability likely constrains the response of plants to elevated CO 2 and the ability of ecosystems to sequester carbon. Because herbivore growth and reproduction scale with protein intake, declining foliar [N] may be contributing to widely reported declines in insect populations and may be negatively affecting the growth of grazing livestock and herbivorous wild mammals. Spatial and temporal patterns in N availability are not yet fully understood, particularly outside of Europe and North America. Developments in remote sensing, accompanied by additional historical reconstructions of N availability from tree rings, herbarium specimens, and sediments, will show how N availability trajectories vary among ecosystems. Such assessment and monitoring efforts need to be complemented by further experimental and theoretical investigations into the causes of declining N availability, its implications for global carbon sequestration, and how its effects propagate through food webs. Responses will need to involve reducing N demand via lowering atmospheric CO 2 concentrations, and/or increasing N supply. Successfully mitigating and adapting to declining N availability will require a broader understanding that this phenomenon is occurring alongside the more widely recognized issue of anthropogenic eutrophication. Intercalibration of isotopic records from leaves, tree rings, and lake sediments suggests that N availability in many terrestrial ecosystems has steadily declined since the beginning of the industrial era. Reductions in N availability may affect many aspects of ecosystem functioning, including carbon sequestration and herbivore nutrition. Shaded areas indicate 80% prediction intervals; marker size is proportional to the number of measurements in each annual mean. Isotope data: (tree ring) K. K. McLauchlan et al. , Sci. Rep. 7 , 7856 (2017); (lake sediment) G. W. Holtgrieve et al. , Science 334 , 1545–1548 (2011); (foliar) J. M. Craine et al. , Nat. Ecol. Evol. 2 , 1735–1744 (2018)« less
3. Transmitted Cross-Polarized Light Detection of Particulate Inorganic Carbon Concentrations and Fluxes in the Ocean Water Column: Ships to ARGO Floats

   https://doi.org/10.3389/frsen.2022.837938  

   Bishop, James K. ; Amaral, Vinicius J. ; Lam, Phoebe J. ; Wood, Todd J. ; Lee, Jong-Mi ; Laubach, Allison ; Barnard, Andrew ; Derr, Alex ; Orrico, Cristina ( March 2022 , Frontiers in Remote Sensing)

   Particulate inorganic carbon (PIC) plays a major role in the ocean carbon cycle impacting pH, dissolved inorganic carbon, and alkalinity, as well as particulate organic carbon (POC) export and transfer efficiency to the deep sea. Remote sensing retrievals of PIC in surface waters span two decades, yet knowledge of PIC concentration variability in the water column is temporally and spatially limited due to a reliance on ship sampling. To overcome the space–time gap in observations, we have developed optical sensors for PIC concentration and flux that exploit the high mineral birefringence of CaCO 3 minerals, and thus enable real-time data when deployed operationally from ship CTDs and ARGO-style Carbon Flux Explorer floats. For PIC concentrations, we describe a fast (10 Hz) digital low-power (∼0.5 W) sensor that utilizes cross-polarized transmitted light to detect the photon yield from suspended birefringent particles in the water column. This sensor has been CTD-deployed to depths as great as 6,000 m and cross-calibrated against particulates sampled by large volume in situ filtration and CTD/rosettes. We report data from the September–November 2018 GEOTRACES GP15 meridional transect from the Aleutian Islands to Tahiti along 152°W where we validated two prototype sensors deployed on separate CTD systems surface to bottom atmore »39 stations, many of which were taken in nearly particle-free waters. We compare sensor results with major particle phase composition (particularly PIC and particulate aluminum) from simultaneously collected size-fractionated particulate samples collected by large volume in situ filtration. We also report results from the June 2017 California Current Ecosystem-Long Term Ecological Research (CCE-LTER) process study in California coastal waters where high PIC levels were found. We demonstrate that the PIC concentration sensor can detect PIC concentration variability from 0.01 to >1 μM in the water column (except in nepheloid layers) and outline engineering needs and progress on its integration with the Carbon Flux Explorer, an autonomous float.« less
4. High-resolution observations of the North Pacific transition layer from a Lagrangian float

   https://doi.org/10.1175/JPO-D-21-0032.1  

   Kaminski, Alexis K. ; D’Asaro, Eric A. ; Shcherbina, Andrey Y. ; Harcourt, Ramsey R. ( August 2021 , Journal of Physical Oceanography)

   Abstract Acrucial region of the ocean surface boundary layer (OSBL) is the strongly-sheared and -stratified transition layer (TL) separating the mixed layer from the upper pycnocline, where a diverse range of waves and instabilities are possible. Previous work suggests that these different waves and instabilities will lead to different OSBL behaviours. Therefore, understanding which physical processes occur is key for modelling the TL. Here we present observations of the TL from a Lagrangian float deployed for 73 days near Ocean Weather Station Papa (50°N, 145°W) during Fall 2018. The float followed the vertical motion of the TL, continuously measuring profiles across it using an ADCP, temperature chain and salinity sensors. The temperature chain made depth/time images of TL structures with a resolution of 6cm and 3 seconds. These showed the frequent occurrence of very sharp interfaces, dominated by temperature jumps of O(1)°C over 6cm or less. Temperature inversions were typically small (≲ 10cm), frequent, and strongly-stratified; very few large overturns were observed. The corresponding velocity profiles varied over larger length scales than the temperature profiles. These structures are consistent with scouring behaviour rather than Kelvin-Helmholtz-type overturning. Their net effect, estimated via a Thorpe-scale analysis, suggests that these frequent small temperaturemore »inversions can account for the observed mixed layer deepening and entrainment flux. Corresponding estimates of dissipation, diffusivity, and heat fluxes also agree with previous TL studies, suggesting that the TL dynamics is dominated by these nearly continuous 10cm-scale mixing structures, rather than by less frequent larger overturns.« less
5. Cloud Microphysical Effects of Aerosol Particle Sources and Marine Boundary Layer Processes

   Sanchez, Kevin J ( January 2017 , UC San Diego Electronic Theses and Dissertations)

   Marine boundary layer (MBL) clouds are an important, though uncertain, part of Earth’s radiative budget. Previous studies have shown sources of aerosol particles in the remote MBL consist of primary sea spray, the oxidation of organic and inorganic vapors derived from the ocean, entrainment from the free troposphere, and anthropogenic pollution. The potential for these particles to become cloud condensation nuclei (CCN) varies largely dependent on their hygroscopic properties. Furthermore, when clouds form, physical processes can alter the optical properties of the cloud. This dissertation aims to identify variations in aerosol sources that affect MBL CCN concentrations and physical processes throughout the cloud lifetime that influence cloud optical properties. Ambient measurements of marine particles and clouds were made throughout two campaigns in the north Pacific and four campaigns in the north Atlantic. Both clean marine and polluted clouds were sampled. In addition, dry MBL particles were measured to identify their chemical composition and size distribution, which is necessary to identify their potential to be CCN active. The organic hygroscopicity influenced CCN concentrations and cloud optical properties significantly for particles that were mostly organic, such as ship stack and generated smoke particles. For a typical range of organic hygroscopicity the amountmore »of reflected solar radiation varied by 2-7% for polluted conditions and less than 1% for clean conditions. Simulated droplet spectral width was shown to be more representative of observations when using a weighted distribution of cloud base heights and updraft velocities, and increased the cloud reflectivity up to 2%. Cloud top entrainment and decoupling of the MBL were found to account for a decrease in cloud radiative forcing. Cloud top entrainment was corrected for homogeneous entrainment and accounted for a decrease in radiative forcing of up to 50 Wm-2. Clustering of individual marine aerosol particles resulted in the identification of particle types derived from dimethyl-sulfide (DMS) oxidation. Two particle types were identified to come from DMS oxidation products and accounted for approximately 25% and 65% of CCN at 0.1% supersaturation during the winter and summer, respectively. One of the particle types was found to be entrained from the free troposphere.« less