Description: We reconstructed a high‐resolution, alkenone‐based sea surface temperature (SST) record spanning the last ca. 150 years, from a sediment core retrieved within the main upwelling zone off Peru. A conspicuous SST decline is evidenced since the 1950s despite interdecadal SST variability. Instrumental SST data and reanalysis of ECMWF ERA 40 winds suggest that the recent coastal cooling corresponds mainly to an intensification of alongshore winds and associated increase of upwelling in spring. Consistently, both proxy and instrumental data evidence increased productivity in phase with the SST cooling. Our data expand on previous reports on recent SST cooling in other Eastern Boundary upwelling systems and support scenarios that relate coastal upwelling intensification to global warming. Yet, further investigations are needed to assess the role of different mechanisms and forcings (enhanced local winds vs. spin‐up of the South Pacific High Pressure cell).

Description: Highlights: • The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century. • This review paper provides a synthesis of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. • Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. • Case studies were selected to achieve a balance in system diversity and global coverage. • Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. • Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. • Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. • Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging. • System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales. Abstract: The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century. This paper provides a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. Case studies were selected to achieve a balance in system diversity and global coverage. Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging. System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.

Description: We reconstructed a high-resolution, alkenone-based sea surface temperature (SST) record spanning the last ca. 150 years, from a sediment core retrieved within the main upwelling zone off Peru. A conspicuous SST decline is evidenced since the 1950s despite interdecadal SST variability. Instrumental SST data and reanalysis of ECMWF ERA 40 winds suggest that the recent coastal cooling corresponds mainly to an intensification of alongshore winds and associated increase of upwelling in spring. Consistently, both proxy and instrumental data evidence increased productivity in phase with the SST cooling. Our data expand on previous reports on recent SST cooling in other Eastern Boundary upwelling systems and support scenarios that relate coastal upwelling intensification to global warming. Yet, further investigations are needed to assess the role of different mechanisms and forcings (enhanced local winds vs. spin-up of the South Pacific High Pressure cell).

Description: This dataset complies carbon and nitrogen isotopic and elemental signature for two dated lacustrine sediment core for the Khnifiss Lagoon, Southern Morocco, for the last century. Chronological data retrieved from 210Pb and 137Cs measurements are also available for these cores. We have also analysed local vegetation specimens, based on the vegetation list of the Khnifiss National Park for their elemental and isotopic analysis of C and N. The cores measuring approximately 50 cm were collected manually from a boat employing a gravity core sampler. The cores were sliced every two centimeters in the field, packed in individual identified plastics bags, and kept refrigerated. The sedimentation rates and chronology of each sediment layer were determined by the CRS (Constant Rate of Supply) model (Appleby & Oldfield, 1978) using natural radionuclides 210Pb and 226Ra and the 1965 bomb peak for 137Cs. The sediment samples were dried in an oven at 50°C for 48 hours, ground in an agate mortar, and stored in sealed disposable plastic Petri dishes. The samples were weighed, and the densities were measured. The high-resolution gamma-ray measurements were performed for 24 hours for each sample using a plane co-axial extended range germanium hyper pure detector (model GX5021 - Canberra), installed at the Department of Physiological Sciences of the Espírito Santo Federal University (UFES), Brazil. Detector relative efficiency is 50%, with a resolution of 2.1 KeV (FWHM) at the 60Co peak (1.33 MeV). Efficiency curves for sample geometry were obtained from a liquid solution containing a mixture of radionuclides (NIST - serial number HV951), including 133Ba, 57Co, 139Ce, 85Sr, 137Cs, 54Mn, 88Y, and 65Zn. Total carbon, nitrogen, and sulfur elemental concentrations were determined with an elemental analyzer Vario El III Elementary (Elementar). Before the δ 13C and Corg determination, an aliquot of the crushed samples was subjected to acid attack (HCl 3%) until the carbonate fraction was removed. Nitrogen and carbon isotopes were analyzed with a FlashHT 2000 elemental analyzer coupled with a Delta V Advantage mass spectrometer from Thermo Fisher Scientific. The δ13C is expressed in per mil (‰) against the international standard VPDB (Vienna Pee Dee Belemnite), and the δ15N in per mil (‰) against Air. The Khnifiss National Park vegetation samples, composed of twigs and leaves, were packed in individual plastic packages and identified to species level, when possible, and later dried in a 50°C oven for 48 hours and ground in an agate mortar at the Faculty of Science, Ibn Zohr University (Agadir, Morocco). Finally, the samples were sent for elemental and isotopic analysis of C and N in the Thermo Fisher Scientific FlashHT elemental analyzer coupled to the Thermo Fisher Scientific Delta V Advantage mass spectrometer. The samples for particle size analysis were treated with 1 N HCl at 25 °C to remove carbonates. Once the digestion process is completed, the samples were washed with distilled water and then were centrifuged at 4000 rpm, after which the supernatant was carefully removed using a Pasteur pipette. After this step, the samples had their organic matter content removed by treatment with concentrated hydrogen peroxide (30%) that was continually added to the sample in a hot plate at 60°C until sample frothing ceased. Thus, only the mineral fraction of the sample remained, presenting no agglutination between the particles. Into this content is was added a dispersant, sodium hexametaphosphate ((NaPO3)6, in the concentration of 40mg/L), thus avoiding the aggregation of particles, which could interfere in the determination of the particles size distribution. The samples were shaken for 24 hours and then analyzed in the CILAS® 1064 Particle Analyzer. The CILAS 1064 has a dual sequenced laser system for a measuring range of 0.04 to 500 µm and delivering the results in 100 interval classes. For the determination of chlorophyll derivatives in sediment, we applied the method described by Sanger et al. (1972). The “sedimentary chlorophyll”, a product of chlorophyll degradation, was extracted from sediments by placing approximately 1g of wet sample to a centrifuge tube, protected from ambient light with laminated paper and by adding 20 mL of 90% acetone in each tube. The tubes were left on a shaking table, in a semi-dark environment, for approximately 20 minutes for two consecutive times, interspersed by centrifugation and removal of the supernatant, and with a third extraction, adding 10 mL of acetone. The absorbance of the extract was measured in a scanning spectrophotometer with a range of 350 to 800 nm. The possible interferences in the absorbance background were corrected by subtracting the chlorophyll peaks from a baseline curve made between 500 nm and 800 nm, thus eliminating the absorbance of non-chlorophyll components (Wetzel, 2001). The concentrations of pigments are expressed as the Sediment Pigment Derivative Unit (SPDU) per gram of organic matter (Vallentyne, 1955). Finally, the concentrations of Sr, Ca, Si e Ti were determined using an Epsilon 3X energy dispersive X-ray fluorescence (EDXRF) spectrometer, PANalytical. A total of 50 samples (26 and 24 for THI and THIII, respectively) were milled into powder and transferred to an open-ended XRF cup covered with 3.6 µm PANalytical thin-film. The reference concentration for each element was based on the concentration of the element, using inter-element slope and baseline-corrected peak heights from the XRF system.

Description: The core was collected manually from a boat employing a gravity core sampler. The core was sliced every two centimeters in the field, packed in individual identified plastics bags, and kept refrigerated. The THIII sediment core (N 28°01.626, W 12°16.558), measuring 48 cm, was recovered in one of the sections of the main water entrance channel in the Lagoon. This site is approximately seven meters deep and lies between two islets of exposed sediment dominated by marine grass specimens of *Zostera noltii*. During the sampling, salinity at the site was 40 PSU (Practical Salinity Unit). Due to its proximity to the inlet, this site is highly influenced by the seawater entrance and ocean nutrients. Aiming the recognition of the environmental changes at this point of the lagoon, we have performed the following measurements in the core's sediments: isotopic ratio of C, dating (210Pb and 137Cs calibration), sedimentation rate, mean grain size, Chlorophyll, Total Organic Carbon (TOC), C/N elemental ratio and C/S elemental ratio.

Description: Aiming at a better recognition of the carbon and nitrogen signatures nature at the different layers in the sediment core in the lake, and avoid using generic ranges for these isotopes, the local vegetation specimens, based on the vegetation list of the Khnifiss National Park, were collected and analyzed for their elemental and isotopic analysis of C and N.

Description: The core was collected manually from a boat employing a gravity core sampler. The core was sliced every two centimeters in the field, packed in individual identified plastics bags, and kept refrigerated. The THI sediment core (N 27°59.139, W 12°17.109), located in the inner portion of the Lagoon, was retrieved from a secondary channel, having 52 cm of length. The salinity measured at the site on the day of the sampling was 50 PSU, thus revealing a saline gradient that increases towards the upstream, probably due to a longer residence time of the water and consequent higher evaporation rate at that site. To its right side, the secondary channel is protected by a desert plateau, while to its left lays a saltmarsh area, occupied by specimens of the genus Salicornia sp., that are periodically exposed by tidal dynamics. A low marine influence characterizes this part of the Lagoon, depths between 2.70 m and 5.20 m, during low and high tides, respectively, and surrounded by extensive heterogeneous marshes. Aiming the recognition of the environmental changes at this point of the lagoon, we have performed the following measurements in the core's sediments: dating (210Pb and 137Cs calibration), sedimentation rate, mean grainsize, Chlorophyll, Total Organic Carbon (TOC), C/N, Sr/Ca and Si/Ti elemental ratios.