Polar Region Bathymetry: Critical Knowledge for the Prediction of Global Sea Level Rise

The ocean and the marine parts of the cryosphere interact directly with, and are affected by, the seafloor and its primary properties of depth (bathymetry) and shape (morphology) in many ways. Bottom currents are largely constrained by undersea terrain with consequences for both regional and global heat transport. Deep ocean mixing is controlled by seafloor roughness, and the bathymetry directly influences where marine outlet glaciers are susceptible to the inflow relatively warm subsurface waters - an issue of great importance for ice-sheet discharge, i.e., the loss of mass from calving and undersea melting. Mass loss from glaciers and the Greenland and Antarctic ice sheets, is among the primary drivers of global sea-level rise, together now contributing more to sea-level rise than the thermal expansion of the ocean. Recent research suggests that the upper bounds of predicted sea-level rise by the year 2100 under the scenarios presented in IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (SROCCC) likely are conservative because of the many unknowns regarding ice dynamics. In this paper we highlight the poorly mapped seafloor in the Polar regions as a critical knowledge gap that needs to be filled to move marine cryosphere science forward and produce improved understanding of the factors impacting ice-discharge and, with that, improved predictions of, among other things, global sea-level. We analyze the bathymetric data coverage in the Arctic Ocean specifically and use the results to discuss challenges that must be overcome to map the most remotely located areas in the Polar regions in general.

Emergence of climate change signals in marine ecosystem thermal niches

Temperature is one of the most important drivers of global ocean patterns of biodiversity1,2,3 shaping thermal niches through thresholds of physiological thermal tolerance4⁠. Because of anthropogenic global warming, lower and upper thermal niche bounds are predicted to change affecting the future distribution of marine species5,6⁠. Current working hypotheses suggest an expansion of ectotherms toward their poleward boundaries7,8. Nonetheless, the knowledge of the timing and extent of these rearrangements across latitude and depth remains limited. Here, using daily data across the water column from both Ocean Sites network observations and novel Earth System Model, we track the emergence of thermal niches whose lower bound is warmer than their current upper bound, potentially disrupting marine habitats. We show that these developments will emerge by ~2030 in subsurface waters (~50 – 1000 m) if anthropogenic emissions continue to rise, whereas they delay several decades if emissions are substantially reduced. By 2100, thermal niches will be warmer than current counterparts. However, we further show that depending on the vertical level, concomitant changes in both boundaries will result in wider or narrower thermal niches. These results suggest that the redistribution of marine species might differ across depth, shedding light upon a much more complex picture of the impact of climate change on marine habitats.

Evidences and drivers of ocean deoxygenation off Peru over recent past decades

Deoxygenation is a major threat to the coastal ocean health as it impacts marine life and key biogeochemical cycles. Understanding its drivers is crucial in the thriving and highly exploited Peru upwelling system, where naturally low-oxygenated subsurface waters form the so-called oxygen minimum zone (OMZ), and a slight vertical shift in its upper limit may have a huge impact. Here we investigate the long-term deoxygenation trends in the upper part of the nearshore OMZ off Peru over the period 1970–2008. We use a unique set of dissolved oxygen in situ observations and several high-resolution regional dynamical-biogeochemical coupled model simulations. Both observation and model present a nearshore deoxygenation above 150 m depth, with a maximum trend of – 10 µmol kg−1 decade1, and a shoaling of the oxycline depth (− 6.4 m decade−1). Model sensitivity analysis shows that the modeled oxycline depth presents a non-significant (+ 0.9 m decade−1) trend when remote forcing is suppressed, while a significant oxycline shoaling (− 3 m decade−1) is obtained when the wind variability is suppressed. This indicates that the nearshore deoxygenation can be attributed to the slowdown of the near-equatorial eastward currents, which transport oxygen-rich waters towards the Peruvian shores. The large uncertainties in the estimation of this ventilation flux and the consequences for more recent and future deoxygenation trends are discussed.

Exploring University-Community Collaborations

The Riverside neighborhood bears multiple burdens of environmental harm. Running the gamut from groundwater contamination in subsurface waters to lead in soils and dust and paint to particulate matter in the air from highways and industry, these environmental insults harm the physical, mental, and economic well-being of the community. The community has also faced an information gap where data was scarce, hard to locate, and sometimes wrong. Activists have long worked to improve the quality of life in the neighborhood, but faced barriers in the form of policies (e.g. Red Lining, zoning variances, disinvestment in public services such as street lights and sidewalks) and practices (e.g. absentee landlords, illegal dumping). Features such as the Central Canal that were developed into recreational amenities in other parts of the city were minimally maintained or restricted from use by residents. In the face of these challenges, IUPUI faculty, students, and community members have partnered on multiple projects to document the history of environmental harms, assess exposure and risk of residents’ exposomes, and share information in ways that are accessible and relevant for residents. The work supports the agency and activism of the community, particularly as it faces pressures of gentrification and university encroachment with the prospect of 16 Tech project expansion. The work also takes place in the context of contested interests and harmful legacies as representatives of an urban university that displaced longtime residents work to partner ethically and transparently with those same communities. As a result, current faculty-community collaborations operate within a space complicated by the problematic legacy of harm and ongoing structural racism. However well-intentioned, faculty, students and community members have to navigate that history and enduring power dynamics as they design their research, identify relevant questions, and share results in ways that are accessible and meaningful to community members.

Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (δ30SidSi) and biogenic silica (δ30SibSi) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More δ30SibSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ30SibSi data (together with δ30SidSi) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The δ30SidSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ30SibSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ30Si) ranging from −0.4‰ to −1.0‰. In contrast, δ30SidSi and δ30SibSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ30SidSi. A strong relationship between δ30SibSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ30SibSi. A comparison of δ30SibSi samples from the water column and from underlying core-top sediments (δ30SibSi_sed.) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ30SibSi_sed. = +0.9‰ to +1.7‰), although we observed small differences in δ30SibSi depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using δ30SibSi data as a paleo proxy for the reconstruction of dSi utilization in the region.

Features of seasonal variability of hydrological and hydrochemical characteristics of subsurface waters of the northwestern Black Sea

In our work, based on the archival data of observations carried out in 1955 – 2015, hydrological and hydrochemical characteristics in the 10–30 m layer in the northwestern shelf (NWS) of the Black Sea, including the Danube estuary area are analyzed. Intra-annual changes in dissolved oxygen and water temperature in the NWS and in the Danube estuary area are shown to have a well-pronounced seasonal character. Water temperature in the estuary area of the Danube is characterized by lower values in all seasons than in the NWS. Salinity at the horizons of 10 and 20 m in winter in the estuary area of the Danube is lower than in the NWS due to the freshening of the upper mixed layer by river runoff. In spring, resulting from increased river runoff, an increase in the stability of water stratification occurs, which prevents the spread of heat inland and vertical exchange of oxygen. The strongest freshening in the studied layer is established at the horizon of 10 m. In spring, the southerly winds “trap” river waters in the shallower part of the shelf, and westerly winds give rise to the spread of freshened waters to the east. The summer-autumn period is characterized by low intensity of vertical and horizontal water exchange, which reduces the flow of oxygen to the subsurface layers. Harmonic analysis shows that in the Danube estuary area and in the NWS, the annual signal is dominant for temperature, salinity and oxygen, except for salinity at the 30 m horizon in the estuary area of the Danube. The semiannual harmonic of salinity at 30 m in the estuarine area of the Danube is most likely related to both limited data availability and their noisiness.

Destruction and reinstatement of coastal hypoxia in the South China Sea off the Pearl River estuary

We examined the evolution of intermittent hypoxia off the Pearl River estuary based on three cruise legs conducted in July 2018: one during severe hypoxic conditions before the passage of a typhoon and two post-typhoon legs showing destruction of the hypoxia and its reinstatement. The lowest ever recorded regional dissolved oxygen (DO) concentration of 3.5 µmol kg−1 (∼ 0.1 mg L−1) was observed in bottom waters during leg 1, with an ∼ 660 km2 area experiencing hypoxic conditions (DO < 63 µmol kg−1). Hypoxia was completely destroyed by the typhoon passage but was quickly restored ∼ 6 d later, resulting primarily from high biochemical oxygen consumption in bottom waters that averaged 14.6 ± 4.8 µmol O2 kg−1 d−1. The shoreward intrusion of offshore subsurface waters contributed to an additional 8.6 ± 1.7 % of oxygen loss during the reinstatement of hypoxia. Freshwater inputs suppressed wind-driven turbulent mixing, stabilizing the water column and facilitating the hypoxia formation. The rapid reinstatement of summer hypoxia has a shorter timescale than the water residence time, which is however comparable with that of its initial disturbance from frequent tropical cyclones that occur throughout the wet season. This has important implications for better understanding the intermittent nature of hypoxia and predicting coastal hypoxia in a changing climate.

Evidences and drivers of ocean deoxygenation off Peru over recent past decades

Deoxygenation is a major threat to the coastal ocean health as it impacts marine life and key biogeochemical cycles. Understanding its drivers is crucial in the thriving and highly exploited Peru upwelling system, where naturally low-oxygenated subsurface waters form the so-called oxygen minimum zone, and a slight vertical shift in its upper limit may have a huge impact. Here we investigate the long-term deoxygenation trends in the upper part of the nearshore oxygen minimum zone off Peru over the period 1970-2008. We use a unique set of dissolved oxygen in situ observations and several high resolution regional dynamical-biogeochemical coupled model simulations. The upper part of the oxygen minimum zone appears to lose oxygen over the period, particularly off Northern Peru, a trend well reproduced by the model. Model simulations attribute the deoxygenation to the slowdown of the near-equatorial eastward currents, which transport oxygen toward the Peruvian shores. The large uncertainties in the estimation of this ventilation flux and the consequences for more recent and future deoxygenation trends are discussed.