CARAPACE: Calcite-Aragonite transition Across Pacific Atolls from the Cretaceous to the Eocene
Lead Research Organisation:
Queen Mary University of London
Department Name: Digital Environment Research Institute
Abstract
We propose a new seagoing research campaign in the Mid Pacific and the Emperor's Seamount Chain regions. Specifically, we are interested in the architecture and geometries of reefs situated atop of ancient volcanoes. In mid-ocean setting, corals and other carbonate producers tend to form significant reefs whenever a shallow substrate is available. As postulated by Darwin in the late 1800's, when the volcanic island sinks into the Earth mantle (a process called subsidence) the reefs keep growing and producing sediments, often forming a carbonate island known as an atoll. Sometimes, atolls can have a lagoon in the middle, where the volcano formerly stood. Interestingly, atolls can live for millions of years, but can also die and drown below the surface of the ocean. Drowned atolls are referred to as "guyots".
A series of guyots in the mid Pacific piqued our interest. There the production of reefs extends from the Early Cretaceous (about 110 million years ago) to the Oligocene (about 30 million years ago) and to the present. Interestingly, the chemistry of the ocean has changed dramatically during this period: in the Early Cretaceous, the ocean was known as a 'calcitic sea', where the mineral calcite was preferentially precipitated. Somewhere between 50-30 million years ago, the chemistry of the oceans changed and we are now in an 'aragonitic sea', where most of the carbonate production is dominated by the mineral aragonite. Exactly why the chemistry of the ocean has changed remains only partially known, but a probable cause is that global climate had fundamentally changed from the Early Cretaceous warm, ice-free, high CO2 concentration world to the cold, punctuated glaciations and low CO2 concentration world of the recent past.
There are two main things we wish to understand with this campaign. The first being the nature of carbonate production across this calcite to aragonite transition, and how the architecture of the carbonate atolls might have adapted in response to this chemical change. Understanding how past carbonates adapted to a different chemistry of the ocean is crucial for predicting how modern climate change will impact corals and other carbonate producers. For instance, will the change in chemistry force carbonate producers such as corals deeper/shallower in the water column? This would impact the geometry of the atoll. Our second objective is to understand by how much global sea-level might have change across the time interval of interest. Current global warming results in the melting of ice sheets at high-latitude, and dramatic sea-level rise. Looking at past example of sea-level changes allows us to calibrate by how much sea-level rises during global warming events. Quantifying the rate of sea-level change can be achieved in carbonates because the reef is growing very close to sea-level, so if we can track the position of the reef and how it changes through time, we can reconstruct global sea-level. For the Cretaceous to Eocene (our window of interest), this is poorly understood.
To achieve our two main objectives, we need to collect data on 6 different drowned atolls (guyots); this ensures that we cover the entire geological period of interest, as each atoll is of a slightly different age. The primary data we will use in this research is known as seismic reflection: we send small seismic waves towards the bottom of the sea, and by reconstructing the time of arrival of the reflected seismic waves we can reconstruct the architecture of the atoll.
In the future, we plan to select a few 'best' targets based on our data to go back to these atolls and sample the rocks using deep-sea scientific drilling.
A series of guyots in the mid Pacific piqued our interest. There the production of reefs extends from the Early Cretaceous (about 110 million years ago) to the Oligocene (about 30 million years ago) and to the present. Interestingly, the chemistry of the ocean has changed dramatically during this period: in the Early Cretaceous, the ocean was known as a 'calcitic sea', where the mineral calcite was preferentially precipitated. Somewhere between 50-30 million years ago, the chemistry of the oceans changed and we are now in an 'aragonitic sea', where most of the carbonate production is dominated by the mineral aragonite. Exactly why the chemistry of the ocean has changed remains only partially known, but a probable cause is that global climate had fundamentally changed from the Early Cretaceous warm, ice-free, high CO2 concentration world to the cold, punctuated glaciations and low CO2 concentration world of the recent past.
There are two main things we wish to understand with this campaign. The first being the nature of carbonate production across this calcite to aragonite transition, and how the architecture of the carbonate atolls might have adapted in response to this chemical change. Understanding how past carbonates adapted to a different chemistry of the ocean is crucial for predicting how modern climate change will impact corals and other carbonate producers. For instance, will the change in chemistry force carbonate producers such as corals deeper/shallower in the water column? This would impact the geometry of the atoll. Our second objective is to understand by how much global sea-level might have change across the time interval of interest. Current global warming results in the melting of ice sheets at high-latitude, and dramatic sea-level rise. Looking at past example of sea-level changes allows us to calibrate by how much sea-level rises during global warming events. Quantifying the rate of sea-level change can be achieved in carbonates because the reef is growing very close to sea-level, so if we can track the position of the reef and how it changes through time, we can reconstruct global sea-level. For the Cretaceous to Eocene (our window of interest), this is poorly understood.
To achieve our two main objectives, we need to collect data on 6 different drowned atolls (guyots); this ensures that we cover the entire geological period of interest, as each atoll is of a slightly different age. The primary data we will use in this research is known as seismic reflection: we send small seismic waves towards the bottom of the sea, and by reconstructing the time of arrival of the reflected seismic waves we can reconstruct the architecture of the atoll.
In the future, we plan to select a few 'best' targets based on our data to go back to these atolls and sample the rocks using deep-sea scientific drilling.
