How salty was the Mediterranean Outflow Water?

Project Information

HowMOW

Grant agreement ID: 101104537

DOI

10.3030/101104537

Project closed

EC signature date 20 April 2023

Start date 15 December 2023

End date 14 December 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 172 618,56

Coordinated by

CENTRO DE CIENCIAS DO MAR DO ALGARVE
Portugal

Periodic Reporting for period 1 - HowMOW (How salty was the Mediterranean Outflow Water?)

Reporting period: 2023-12-15 to 2025-12-14

Summary of the context and overall objectives of the project

Climate simulations suggest a collapse of the ocean circulation in the near future, resulting in conditions similar to past climates as reported by Intergovernmental Panel on Climate Change (IPCC) based on ample geological evidence. The THC incorporates density-driven vertical mixing process controlled by two major variables: temperature and salinity. While temperature is more influential in the surface ocean, salinity controls the deep ocean. Higher salinities enhance the THC, whereas lower salinities cause a slowdown or collapse of the THC, as is expected by the end of the 21st century due to ice melting. Despite the importance of deep ocean salinity in glacial-interglacial variations as models suggest, we lack a direct salinity proxy to reveal its role in past climates to understand future conditions. Here we ask: if we do not know the past ocean salinities and THC dynamics, how can we confidently rely on THC-related climate analogs for future conditions? To evaluate processes that impacted the THC with lower or higher salinities in the past, we urgently need to develop an independent and direct proxy for salinity, which is the objective of this project. Therefore, the sole objective of HowMOW is to explore a new direct and independent salinity proxy. To explore this new proxy, trace element analysis on fossil foraminifers, unicellular calcareous marine organisms, is applied by using advanced geochemical methods. Sodium/calcium values of fossil foraminifer shells are expected to reflect seawater salinities.

The ultimate goal of this project is to explore the role of salinity variation in the deep thermohaline circulation during the major climate shifts in the geological record. Towards achieving this goal, an important location is chosen: the Gulf of Cadiz. The Gulf of Cadiz is the location where the saltiest water mass, Mediterranean Outflow Water, exits the Mediterranean Sea and joins the Atlantic Ocean, and thus it is hypothesised that it played a significant role in THC variation during important climate shifts by being a salt injector to the THC. To test the efficacy of this new proxy, an important climate shift with ample data to compare in the study area is chosen: the Early-Middle Pleistocene Transition (EMPT). The EMPT resulted in the replacement of symmetric 41-kyr interglacial-glacial cycles with 100-kyr abrupt and asymmetric cycles about 1.2-07 million years ago. If deep-sea benthic foraminiferal Na/Ca is a proxy in paleoclimate studies, it should reflect a similar variation as obtained from continuous sediment cores recovered from the Gulf of Cadiz.

HowMOW relates to Sustainable Development Goals 13 and 14 of United Nations, which are Climate Action and Life Below Water, respectively. Climate Action aims at reducing the impact of global warming. Accordingly, "tropicalization" of the Mediterranean Sea is an ongoing phenomenon, which is described as the increase in tropic and alien invasive species in the region linked to increased temperature and salinity of the seawaters linked to global warming. Increase of non-endemic biota is adversely affecting the marine ecosystem of the Mediterranean Sea, altering the marine food web and eventually fisheries industry. Furthermore, it is suggested that mass mortality of the endemic bivalve Pinna nobilis, which is an endangered species listed by the International Union for Conservation of Nature (IUCN), is caused by a pathogen which thrives in increased salinity and high temperatures, linked to global warming. The proxy aimed at within HowMOW directly answers to one of the governing parameters of the tropicalization phenomenon and the habitable ecosystem of endemic species. The outcomes of HowMOW will potentially open doors into a more detailed research to reveal the onset of tropicalization, combined with other geochemical parameters contributing to scientific advancement, providing a new tool in paleoclimate research. Once proven for geological time scales, it can be applied for decadal time scales to research the impacts of global warming with associated parameters towards appropriate adaptation and mitigation acts providing essential information to policymakers.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

HowMOW project involved several laboratory work steps:
1) Sample picking: From 73 seabottom samples, 2-10 specimens of Cibicides pachyderma and/or Planulina ariminensis, when present, were handpicked under a binocular microscope at the Optical Laboratory facilities at the host institute. The sample set consisted of material from the Gulf of Cadiz and the Iberian Margin.

2) Sample cleaning: Prior to trace element analysis, the specimens need to be cleaned in the Clean Laboratory facilities available at IPMA. The traditional procedure for foraminifer cleaning is performed in three steps: (1) clay cleaning by pure water and methanol/ethanol rinsing; (2) reductive cleaning to remove secondary manganese-iron oxides; and (3) oxidative cleaning to remove organic residues. The third step requires the use of a reagent that consists of sodium (NaOH), which may bias the original data by overestimating sodium values. For this reason, to understand the best practice to extract sodium data from foraminifer tests and develop a standardize cleaning protocol, the PI has performed an experiment using three different reagents at the oxidative step. Accordingly, a new routine has been designed, and the traditional use of NaOH has been abandoned and replaced with NH4OH.

3) Trace element analysis: A dataset from 73 seabottom samples has been produced by applying trace element analysis (instrument: Triple-Q ICP-MS at CCMAR Clean Laboratory Facilities).

4) Establishing the species-specific calibration equation: Currently, post-analysis data reduction work is being carried out. Upon finalizing the data reduction, species-specific calibration equations for C. pachyderma and P. ariminensis will be produced. To produce these equations, modern-day salinity data have been collected from the same or neighboring stations. The calibration will be generated to mathematically represent the relationship between Na/Ca content of the foraminifer shells and the salinity of the ambient waters. A dataset will be prepared and shared on open-access platforms, following FAIR principles, preferably PANGAEA (alternatively Zenodo), without embargo.

5) Implementation of calibration formula to fossil material: A dataset from 260 downcore samples of handpicked C. pachyderma and P. ariminensis from IODP site U1387 has been produced by applying trace element analysis (instrument: Triple-Q ICP-MS at CCMAR Clean Laboratory Facilities) following the updated cleaning protocol. Upon the finalization of the data reduction and establishment of species-specific calibration equations, these equations will be applied by using Na/Ca values obtained from trace element analysis and salinities will be calculated at each sample level. Samples are specifically chosen to cover every ~4-10 kiloyear intervals to obtain a good dataset from 41 and 100 kiloyear cycles to observe whether this pronounced climate shift, i.e. Early Middle Pleistocene Transition, is also mirrored in terms of salinity of the Mediterranean Outflow Water.

6) Merging the new and previous datasets: This step requires a comparison of the new dataset with the previously generated stable oxygen isotope dataset from benthic foraminifers of the same species.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Despite the ongoing work on the dataset obtained and hence final results cannot be fully described here, some significant improvements can be shared:

1) As part of the HowMOW project, a significant improvement in laboratory practices for a broader scientific interest has been made, and an updated cleaning protocol has been designed for obtaining foraminiferal Na/Ca values via trace element analysis, especially when only a small quantity of sample can be used, and hence, a minimum contamination is a must.

2) Initial data towards establishing a calibration equation show promising results, with a good correlation coefficient of R=0.6; however, a final statement can be made only once the data reduction is completed and sodium values are plotted against ambient salinity values.

3) Our preliminary insight from specific glacial and interglacial cycles, without having produced the calibration equations yet, reveals a promising result where foraminiferal Na/Ca values follow a similar trend as the benthic foraminiferal oxygen isotope data.

Study area, sample locations, circulation of the main water masses, ansd salinity transect of MOW.