They also reconstructed the response of plant communities and part of the phytoplankton to this crisis.
Iván Rodríguez-Barreiro and José Bienvenido Diez, researchers from the BASAN (XM-3) group at CIM, were responsible for developing this study alongside international scientists.
Oceanic anoxic events (OAEs) were significant oceanographic crises related to long-term disruptions in the global carbon cycle during the Cretaceous. One of the most well-known is OAE2, recorded worldwide during the Cenomanian-Turonian boundary with an estimated duration of between 0.5 and 1 million years. The climatic and oceanographic changes associated with OAE2 disrupted terrestrial and marine ecosystems globally, characterized by extreme climatic conditions, high atmospheric CO2 levels, abnormally high temperatures, and sea levels at their highest point of the Cretaceous due to the lack of significant continental ice volume.
To understand the real impact of this event, a group of researchers from various international institutions conducted research focused on the Vigo Seamount, the results of which were recently published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. The study was developed by Iván Rodríguez-Barreiro and José Bienvenido Diez, researchers from the BASAN (XM-3) group at the Marine Research Center of the University of Vigo, along with other scientists from the Swedish Museum of Natural History; the National Autonomous University of Mexico (UNAM); the University of Ghent; and Oxford University. As explained by Rodríguez-Barreiro, the main author of the research, “a palynological and geochemical analysis of the Cretaceous deposits of the Vigo Seamount” was conducted, obtaining “interesting” results such as the sedimentary record of the extreme OAE2 event and a reconstruction of the terrestrial ecosystems in what would now correspond to Galicia and northern Portugal. Additionally, this work is particularly relevant as it is the first time such an analysis has been done for this period, and it will form part of Rodríguez-Barreiro’s doctoral thesis, which is about to be presented.
Confirmed presence of OAE2 in the Vigo Seamount sedimentary record
As the researchers report, the mid-Cretaceous period experienced significant environmental changes due to extreme climatic conditions, pronounced volcanism, and the distribution of continents. In the marine realm, these factors led to a lack of oxygen at the bottom of the ancient Atlantic Ocean, known as Anoxic Event 2 (OAE2), one of the most disruptive events for the ecosystems of that time. These events are recognized in the sedimentary record as a carbon isotope anomaly and, while it is known that marine communities were affected by these events, the response of terrestrial communities has been little investigated. The study led by UVigo focuses on the Vigo Seamount, located beyond the Iberian continental shelf, along with the Porto Seamount, Vasco de Gama Seamount, and the best-known, the Galicia Bank. These seamounts, details Iván Rodríguez, are about 150 km from the Galician coast and, in the case of the Vigo Seamount, would draw a straight line from the Portuguese city of Porto.
The international team studied the deposits collected under the Deep Sea Drilling Project, a major initiative in the fields of geology and oceanography that drills into the seabeds around the world with different objectives depending on the campaign. In this case, the samples correspond to campaign 47B, carried out in the late 1970s, which studied the southern flank of the Vigo Seamount.
The main objective of the current study led by UVigo was to confirm the presence of the OAE2 event in the Vigo Seamount sedimentary record, “something that had been suspected for many years but no one had applied a carbon-13 isotopic analysis,” the main method for identifying anoxic events, as these crises record a notable anomaly due to their effect on the global geochemical carbon cycle. The other objective, if the first was confirmed, was to study the response of plant communities and part of the phytoplankton, specifically dinoflagellates, to this event.
The results obtained corroborate the initial premises, as the isotopic analysis, carried out at CACTI, confirms the presence of the event. For Rodríguez-Barreiro, “the most important aspect of the results is the response observed from different biological groups.” On the one hand, phytoplankton was negatively affected with a drastic reduction in the abundance and diversity of dinoflagellates as soon as OAE2 began. In the case of plant communities, a very interesting floristic change was observed, as during OAE2 the group Normapolles (pioneers of Fagales, trees with fruits like walnuts) became the dominant part of the forests. This response had also been observed in other parts of southern Europe during an intermediate phase of OAE2, but at the Vigo Seamount, it is recorded a bit earlier, right at the beginning of the event, and more pronounced. Today, the dominance of angiosperms in terrestrial ecosystems is normal, but the first time this happens in the history of our planet is in the mid-Cretaceous and everything seems to point to an extreme climatic event like the one triggered by OAE2 playing an important role.
First reconstruction of terrestrial ecosystems for this period
As part of this research, and to analyze the response of plant communities, the team also reconstructed the ecosystems corresponding to the moments before and during the OAE2 crisis.
This task was performed based on the palynological content (spores, pollen, and dinoflagellate cysts, among others) of the Vigo Seamount sediments. As Rodríguez explains, “when these sediments were deposited, the seamount was at a lesser depth, but it was still a deep marine environment. However, some terrestrial elements such as spores and pollen were transported by wind and water currents to the location of the Vigo Seamount. The nearest coast was that of present-day Galicia and northern Portugal, so we are sure that these elements represented the plant communities of this region at that time,” he explains. Thus, the researchers observe flora mainly composed of “medium and tall conifers (families Pinaceae, Podocarpaceae, and Cheirolepidiaceae) and ferns, both herbaceous and tree ferns (families Anemiaceae and Cyatheaceae), which fluctuate in dominance depending on humid and dry climatic periods.” This is a trend, they add, “that remains more or less stable between the Late Jurassic and Early Cretaceous (for about 60 million years) but introduces a new element (angiosperms represented by the Normapolles group) during OAE2 and which will be very relevant in the following 40 million years.”
Source: DUVI