(1) Portuguese Institute for the Sea and Atmosphere (IPMA), Lisbon, Portugal
(2) Centre for Marine Sciences (CCMAR), University of the Algarve
The ocean is not only vast, but also complex. Nowadays, we cannot perceive it exclusively as a part of the Earth's hydrosphere - although this notion alone makes the ocean a complex system, consisting of water masses with specific physical and chemical properties, which move in different directions and at different depths.
The ocean is the most important subsystem of the climate system, since it covers about 71% of the earth's surface; its surficial and deep circulation distribute energy throughout the planet; produces close to 50% of the global primary production, which makes it the source of a large amount of oxygen; supports a great biodiversity. It contains 60 times more carbon than the atmosphere, and it holds a higher thermal capacity than the atmosphere. In this sense, it plays a very important role in regulating the climate and the carbon and nutrient cycles that are crucial to the organisms that form the basis of the ocean food chain, the primary producers.
Among the anthropogenic risks, the most important are those caused by the increase in the concentration of greenhouse gases in the atmosphere. The discovery that the CO2 concentration in the atmosphere was increasing happened in the early 1970s from the analysis of the records of the Mauna Loa Observatory in Hawaii (Keeling, 1960). After this unveiling and the scientific community's understanding of the disastrous consequences of this phenomenon for the biosphere, the paleosciences community (Palaeoceanography/Paleoclimate) immediately understood that it was necessary to verify whether the phenomenon was recent or just a repetition of some event that had already happened in the geologic past.
The study of the concentration of conserved CO2 in the air bubbles that are trapped in the ice sheets deposited annually in the Arctic and Antarctic revealed that the exponential increase in concentrations began around 1850, at an unprecedented rate over the last 125.000 years, proving in 1987 (Barnola et al., 1987) that urgent action was necessary. Today, the ice record reaches 800.000 years, and the data remains correct.
The ocean absorbed more than 90% of the excess energy accumulated by the climate system between 1971 and 2010, and about 30% of the anthropogenic CO2 emitted in the same period, which causes an increase in energy and heat in the ocean waters, a decrease in oxygen concentration and the ocean water acidification (Sabine et al., 2004). Changes that have a deep environmental impact and are of great concern, because they can result in the decline of a large part of the microorganisms (primary producers) that are at the base of the marine food chain (Feely, 2004). This process will affect the most important sources of fishing in the world, which are concentrated in about 1% of the ocean - with Portugal being part of this area (Fiúza, 1983; Barton et al., 1998). All fisheries - particularly sardines - could suffer from these environmental changes.
Understanding the origin of current changes in the ocean, to anticipate the future, implies a better understanding of the ocean's interior - about which one has very little knowledge. Modern ocean studies range from the use of satellites to direct sampling studies through human diving, vessels or research vessels with mechanical drilling towers that can operate in up to five km of water column, and drill several hundred of meters from below the ocean floor. The studies of seafloor rocks provide information on the frequency of major earthquakes, tsunamis, meteoritic impacts, biological extinctions, and climatic variations.
Figure 1: The ship Joides Resolution, working for the IODP (International Ocean Discovery Program), in the port of Lisbon in October 2022, with the group of researchers who participated in Expedition 397 – Iberian Margin Paleoclimate. (Credit: Sandra Herrmann, IODP, JRSO)
The recent technological developments allow for the installation of observatories on the ocean floor that also record information along the entire water column; together with data collected by a whole range of small robotic autonomous vehicles, which collect data at different ocean depths, will warrant a significant progress in this domain (Favali et al., 2015). Moreover, the commitment of the international community to uphold the principles of "knowing to preserve" and "acquiring data once and using it many times" will certainly contribute to accelerate knowledge and better preserve the environment.
Just like the trees, which record the environmental conditions that occur annually in their tree rings allowing high temporal resolution climatic reconstructions, the marine sediments that accumulate on the ocean floor permit the same type of reconstruction but with lower temporal resolution. However, they constitute the only records from which we can study the climatic variability during older periods (from hundreds to millions of years).
Continuous and long data series constitute a consensual support to evaluate the behaviour of natural systems, e.g., seismicity and tsunamis (Miranda et al., 2015). Estimating the impact of humanity's interaction on the climate system involves generating time series beyond instrumental data and using a geological retrospective from the past, thus allowing to understand the present and anticipate the future.
The sediments of the Portuguese continental margin are crucial for the reconstruction of past climate, as they simultaneously record the millennial climatic variability occurred in the Arctic and Antarctica during the Quaternary (last 2.6 milhões de anos). Hence, they have the potential to extend this record to times prior to the ones recorded in the polar ice caps. Moreover, in addition to the reconstruction of the climate for long periods (hundreds of thousands to millions of years (e.g., Rodrigues et al, 2017; Hodell, Abrantes et al., 2023); they also provide valuable information about the last 2.000 years (e.g., Abrantes et al, 2005, 2011, 2017), or even extreme weather events (e.g., Salgueiro et al., 2010; Naughton et al., 2021).
Despite the desire to preserve pristine or untouchable natural systems, the exploitation of non-living resources in the oceans is urgent due to the extensive occupation of the territory by societies and to political conflicts. Hence, and to address these risks, simultaneously with the development of exploratory capacity and effort, through direct bottom sampling platforms and indirect methods, several endeavours are taking place with the objective of understanding how the impact of the exploration of mineral occurrences on the ocean bottom can impact the general ocean environment and the deep ecosystems in particular (Silva et al., 2023).
Figure 2: Emília Salgueiro (sedimentologist) describes a sedimentary sequence just collected. IODP Exp397 Iberian Margin Paleoclimate (Credit: Sandra Herrmann, IODP, JRSO)
Figure 3: (co-chief) at a results presentation meeting during the IODP Exp397 Iberian Margin Paleoclimate. (Credit: Sandra Herrmann, IODP, JRSO)
Figure 4: Joides Resolution Platform where drilling of sediments from under the ocean floor takes place. IODP Exp397 Iberian Margin Paleoclimate. (Credit: Sandra Herrmann, IODP, JRSO)
References
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