Water mass variability in the eastern South Pacific and the ventilation of the oxygen minimum zone

This dissertation aims at extending our knowledge of the ESP OMZ through two main approaches. The first approach is based on tracer analysis and inverse modeling techniques. Such techniques are applied to hydrographic datasets in order to examine the water mass structure, its variability and its role on the ventilation of the ESP OMZ. The second approach brings a more dynamical perspective to the study of the ventilation of the ESP OMZ, with the assessment of the annual-mean advective and turbulent oxygen transports into the ESP OMZ from Argo and dissolved oxygen datasets. We describe the distribution of water masses offshore Chile, Peru and Ecuador and discuss their spreading pathways. A thorough characterization of the water masses has been accomplished, resulting in the first set of ESP water types accounting for inorganic nutrients and dissolved oxygen. The low oxygen waters that compose the ESP OMZ are mainly Equatorial Subsurface Water (ESSW). The ESP OMZ is ventilated from the south by the Antarctic Intermediate Water (AAIW) and by the shallower Subantarctic Water (SAAW). These water masses are transported into this region by the Peru Chile Current (PCC) or by the adjacent flow that forms part of the subtropical gyre. We also examine the changes induced by two opposite phases of ENSO in water mass distribution and biogeochemical activity. During La Niña, vigorous upwelling promotes the rise in depth of the upper part of the ESP OMZ and denitrification strengthens in the subsurface layer. Furthermore, the upward displacement of isopycnals induced by La Niña favors the ventilation of a different depth range of the OMZ by the upper portion of the AAIW. The opposite occurs during El Niño conditions. We find that, with a larger oxygen supply, respiration increases balancing most of the extra oxygen gain. This suggests that there is an excess of organic matter waiting to be remineralized whenever oxygen supply increases slightly and this situation favors the maintenance of the ESP OMZ. Furthermore, our results show that AAIW flowed along shallower isopycnals in 2009 than in 1993. Such shoaling is not caused by the ENSO phenomenon and changes the way AAIW ventilates the ESP OMZ. This finding might explain why an increase in oxygen content has been reported between 200 and 700m off Chile. A global picture of the main processes and predominant paths of oxygen supply into ESP OMZ is provided here for the first time. Two main advective routes are found, the traditional equatorial pathway and a previously unreported subtropical pathway. Remarkably, the subtropical pathway provides more net oxygen gain than the equatorial pathway at the core of the ESP OMZ. This finding challenges the common assumption that the ESP OMZ is only ventilated by the eastward flowing zonal currents of the Equatorial Current System. This result is endorsed by the independent water mass analyses accomplished in the first part of this dissertation where the fingerprint of AAIW and SAAW is clearly found in the ESP OMZ. In addition, an unreported eastward zonal current, located at intermediate depths between 12-15ºS, is found flowing all the way from the central Pacific into the ESP OMZ and its oxygen supply is quantified. Mean advection dominates oxygen supply in the upper layers due to the large supply by the eastward flowing equatorial zonal currents. However, epineutral turbulent diffusion becomes the dominant term at levels deeper than ¿n = 26.75 kg/m3. The annual mean oxygen budget for the whole volume of the ESP OMZ is unveiled. Epineutral turbulent diffusion provides a net oxygen supply of 417.4 ± 43.0 kmol/s, advection supplies 292.7 ± 25.2 kmol/s and dianeutral turbulent diffusion provides 85.8 ± 17.6 kmol/s. The mean biological consumption of oxygen required to close the budget is 795.9 ± 195.0 kmol/s.El Pacífico Suroriental (ESP) es una región clave del océano con influencia directa sobre el clima global. Dos claros ejemplos son el fenómeno de El Niño-Oscilación del Sur (ENSO), el cual sucede en el Pacífico pero altera el tiempo atmosférico a nivel mundial, y el reciente descubrimiento de que la ralentización observada desde el 2001 en el incremento de la temperatura media mundial se debe parcialmente a un fortalecimiento del afloramiento de aguas frías en el ESP. Durante la última década la comunidad científica ha puesto su atención en las zonas de mínimo de oxígeno (OMZs) del océano global, estando una de las más intensas localizada en el ESP. Las OMZs parecen estar expandiéndose con el cambio climático, esto amenaza la existencia de productivos hábitats marinos a la vez que provoca una pérdida del nitrógeno oceánico, y la pérdida de este nutriente debilita la productividad primaria oceánica y por tanto la capacidad del océano para secuestrar dióxido de carbono. Dentro de las OMZs con núcleo subóxico, el nitrógeno disuelto es degradado y liberado a la atmósfera en forma de varios gases, uno de ellos es un gas de potente efecto invernadero, el óxido nitroso, y su emisión altera aún más el balance radiativo de la atmósfera. Esta tesis tiene como objetivo principal avanzar en el conocimiento de la OMZ del ESP a través de dos estrategias principales. La primera estrategia está basada en análisis de trazadores y en el uso de modelos inversos. Estos modelos han sido aplicados a datos hidrográficos con el objetivo de averiguar la distribución de masas de agua, su variabilidad y su papel en la ventilación de la OMZ del ESP. En la segunda estrategia se estudia la ventilación de la OMZ del ESP desde la perspectiva de la dinámica, mediante el cálculo de los transportes promedios anuales de oxígeno por advección y por turbulencia a partir de datos de oxígeno disuelto y de perfiladores Argo. En esta tesis se describe la distribución de masas de agua del océano abierto frente a Chile, Perú y Ecuador y se discuten las rutas que siguen al esparcirse por el océano. Para ello se ha realizado una caracterización rigurosa las propiedades (valores tipo) de las mismas. Se proporciona aquí el primer conjunto de valores tipo de nutrientes y oxígeno disuelto para las masas de agua presentes en el ESP. El agua pobre en oxígeno que constituye la OMZ del ESP es principalmente Agua Ecuatorial Subsuperficial (ESSW). Dicha OMZ se ventila desde el sur con Agua Antártica Intermedia (AAIW) y, a niveles más someros, con Agua Subantártica (SAAW). Estas masas de agua llegan a esta región transportadas por la Corriente de Perú-Chile (PCC) o por el flujo adyacente que forma parte del giro subtropical situado más al oeste. También se describen los cambios inducidos en la distribución de masas de agua y en la actividad biogeoquímica por fases opuestas del fenómeno ENSO. El vigoroso afloramiento característico de La Niña produce una ascensión en profundidad de la parte superior de la OMZ del ESP a la vez que favorece el proceso de denitrificación en capas subsuperficiales. La Niña induce un desplazamiento hacia capas más someras de las isopicnas y esto favorece la ventilación de un rango de profundidades distinto de la OMZ del ESP con la parte superior de la AAIW. Durante El Niño sucede lo contrario. Por otro lado, se ha observado que la respiración aumenta cuando se incrementa el aporte de oxígeno a la OMZ, balanceando de esta manera la ganancia extra de oxígeno. Esto sugiere que existe un exceso de materia orgánica esperando a ser remineralizada en cuanto se produzca un ligero incremento en el aporte de oxígeno. Este exceso de materia orgánica favorece por tanto la estabilidad de la OMZ del ESP. Nuestros resultados muestran que AAIW fluye en isopicnas más someras en 2009 respecto a 1993 y este cambio no es debido al fenómeno ENSO. Este cambio altera el patrón de ventilación de la OMZ del ESP por AAIW y podría ser la causa del incremento de oxígeno disuelto encontrado recientemente entre 200 y 700 m en el Pacífico chileno. En esta tesis se proporciona la primera imagen global de los principales procesos y rutas de aporte de oxígeno a la OMZ del ESP. Se han encontrado dos rutas principales, la tradicional ruta ecuatorial y la inexplorada ruta subtropical. La ruta subtropical proporciona una mayor ganancia neta de oxígeno que la ruta ecuatorial en el núcleo de la OMZ del ESP. Este hallazgo desafía la asunción generalizada de que la OMZ del ESP se ventila únicamente por el Sistema Ecuatorial de Corrientes. Los análisis de masas de agua realizados en la primera parte de esta tesis de manera independiente respaldan dicho hallazgo al delatar la presencia de AAIW y SAAW en la OMZ del ESP. Por otro lado, se ha descrito por primera vez una corriente que fluye hacia el este en capas intermedias entre 12-15ºS desde el Pacífico central hasta la región de la OMZ del ESP. El aporte de oxígeno de dicha corriente a la OMZ del ESP ha sido cuantificado aquí por primera vez. En lo referente al aporte de oxígeno, la advección es el término dominante en las capas superiores debido al gran aporte de las corrientes ecuaoriales zonales que fluyen hacia el este. Sin embargo por debajo de γn = 26.75 kg m-3, el término dominante es la difusión turbulenta epineutra de oxígeno. El balance medio anual de oxígeno para todo el volumen de la OMZ del ESP es el siguiente. La difusión turbulenta epineutra proporciona una ganancia neta de oxígeno de 417.4 ± 43.0 kmol s-1, la advección aporta 292.7 ± 25.2 kmol s-1 y la difusión turbulenta dianeutra la difusión turbulenta epineutra 85.8 ± 17.6 kmol s-1. Finalmente, el consumo biológico de oxígeno necesario para cerrar es 795.9 ± 195.0 kmol s-1.Postprint (published version

Warming events projected to become more frequent and last longer across Antarctica

Summer temperatures are often above freezing along the Antarctic coastline, which makes ice shelves and coastal snowpacks vulnerable to warming events (understood as periods of consecutive days with warmer than usual conditions). Here, we project changes in the frequency, duration and amplitude of summertime warming events expected until end of century according to two emission scenarios. By using both global and regional climate models, we found that these events are expected to be more frequent and last longer, continent-wide. By end of century, the number of warming events is projected to double in most of West Antarctica and to triple in the vast interior of East Antarctica, even under a moderate-emission scenario. We also found that the expected rise of warming events in coastal areas surrounding the continent will likely lead to enhanced surface melt, which may pose a risk for the future stability of several Antarctic ice shelves

Isotopic Characterization of Water Masses in the Southeast Pacific Region: Paleoceanographic Implications

In this study, we used stable isotopes of oxygen (δ18O), deuterium (δD), and dissolved inorganic carbon (δ13CDIC) in combination with temperature, salinity, oxygen, and nutrient concentrations to characterize the coastal (71°–78°W) and an oceanic (82°–98°W) water masses (SAAW—Subantarctic Surface Water; STW—Subtropical Water; ESSW—Equatorial Subsurface water; AAIW—Antarctic Intermediate Water; PDW—Pacific Deep Water) of the Southeast Pacific (SEP). The results show that δ18O and δD can be used to differentiate between SAAW-STW, SAAW-ESSW, and ESSW-AAIW. δ13CDIC signatures can be used to differentiate between STW-ESSW (oceanic section), SAAW-ESSW, ESSW-AAIW, and AAIW-PDW. Compared with the oceanic section, our new coastal section highlights differences in both the chemistry and geometry of water masses above 1,000 m. Previous paleoceanographic studies using marine sediments from the SEP continental margin used the present-day hydrological oceanic transect to compare against, as the coastal section was not sufficiently characterized. We suggest that our new results of the coastal section should be used for past characterizations of the SEP water masses that are usually based on continental margin sediment samples

Persistent extreme ultraviolet irradiance in Antarctica despite the ozone recovery onset

Attributable to the Montreal Protocol, the most successful environmental treaty ever, human-made ozone-depleting substances are declining and the stratospheric Antarctic ozone layer is recovering. However, the Antarctic ozone hole continues to occur every year, with the severity of ozone loss strongly modulated by meteorological conditions. In late November and early December 2020, we measured at the northern tip of the Antarctic Peninsula the highest ultraviolet (UV) irradiances recorded in the Antarctic continent in more than two decades.The support of INACH (RT_69-20 & RT_70-18), ANID (ANILLO ACT210046, FONDECYT 1191932 & 1221122, DFG190004 and REDES180158), CORFO (19BP-117358 & 18BPE-93920) is gratefully acknowledged

Black carbon in the Southern Andean snowpack

The Andean snowpack is an important source of water for many communities. As other snow-covered regions around the world, the Andes are sensitive to black carbon (BC) deposition from fossil fuel and biomass combustion. BC darkens the snow surface, reduces the albedo, and accelerates melting. Here, we report on measurements of the BC content conducted by using the meltwater filtration (MF) technique in snow samples collected across a transect of more than 2500 km from the mid-latitude Andes to the southern tip of South America. Addressing some of the key knowledge gaps regarding the effects of the BC deposition on the Andean snow, we identified BC-impacted areas, assessed the BC-related albedo reduction, and estimated the resulting snow losses. We found that BC concentrations in our samples generally ranged from 2 to 15 ng g-1, except for the nearly BC-free Patagonian Icefields and for the BC-impacted sites nearby Santiago (a metropolis of 6 million inhabitants). We estimate that the seasonal snowpack shrinking attributable to the BC deposition ranges from 4 mm water equivalent (w.e.) at relatively clean sites in Patagonia to 241 mm w.e. at heavily impacted sites close to Santiago. © 2022 The Author(s). Published by IOP Publishing Ltd

Black carbon in the Southern Andean snowpack

The Andean snowpack is an important source of water for many communities. As other snow-covered regions around the world, the Andes are sensitive to black carbon (BC) deposition from fossil fuel and biomass combustion. BC darkens the snow surface, reduces the albedo, and accelerates melting. Here, we report on measurements of the BC content conducted by using the meltwater filtration (MF) technique in snow samples collected across a transect of more than 2500 km from the mid-latitude Andes to the southern tip of South America. Addressing some of the key knowledge gaps regarding the effects of the BC deposition on the Andean snow, we identified BC-impacted areas, assessed the BC-related albedo reduction, and estimated the resulting snow losses. We found that BC concentrations in our samples generally ranged from 2 to 15 ng g-1, except for the nearly BC-free Patagonian Icefields and for the BC-impacted sites nearby Santiago (a metropolis of 6 million inhabitants). We estimate that the seasonal snowpack shrinking attributable to the BC deposition ranges from 4 mm water equivalent (w.e.) at relatively clean sites in Patagonia to 241 mm w.e. at heavily impacted sites close to Santiago. © 2022 The Author(s). Published by IOP Publishing Ltd

Black carbon footprint of human presence in Antarctica

Black carbon (BC) from fossil fuel and biomass combustion darkens the snow and makes it melt sooner. The BC footprint of research activities and tourism in Antarctica has likely increased as human presence in the continent has surged in recent decades. Here, we report on measurements of the BC concentration in snow samples from 28 sites across a transect of about 2,000 km from the northern tip of Antarctica (62°S) to the southern Ellsworth Mountains (79°S). Our surveys show that BC content in snow surrounding research facilities and popular shore tourist-landing sites is considerably above background levels measured elsewhere in the continent. The resulting radiative forcing is accelerating snow melting and shrinking the snowpack on BC-impacted areas on the Antarctic Peninsula and associated archipelagos by up to 23 mm water equivalent (w.e.) every summer

​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system