Analysing the tidal state of a pre-plate tectonic Earth during the Archean Eon (3.9 Ga)

<p>Deep time investigations of the Earth have revealed a relationship between plate tectonic motion and the intensity of the tide. Tidal energetics change as continental plates disperse and aggregate in the supercontinent cycle, altering ocean basins around them. The question is, could enhanced tides occur on Earth before plate tectonics started e.g., during the Archean?  </p><p>Here we have coupled an established tidal model with an ensemble of potential topographies of the Archean Earth to establish a statistically significant approximation of Archean tidal energetics. Land area is restricted to 5 – 15% with the rest representing primordial ocean – containing no major plate tectonic features i.e., trenches and ridges. Ocean volume is preserved at close to present-day which means oceans are on average 1 km shallower than present-day oceans. Archean day length is set at 13.1 hours with the semi-diurnal tide occurring every 6.8 hours. Equilibrium tide is around 3.4x the present-day value due to the proximity of the Moon.</p><p>The aim of this study is to assess the relationship of the Earth Moon system during this primordial stage to better understand the potential role tides had in the origin of life, and to quantify the tidal state of a primordial rocky planet with a young, nearby moon. Understanding the tidal state of Earth at this early time is important for exoplanetary studies as it broadens our scope of planets which may be hospitable to life.</p><p>We found coastal and open ocean resonance in many of the ensemble topographies. Total global dissipation in the ensembles varies from 75 – 150% of present-day dissipation rates due to elevated equilibrium tide and greater area where the tide can dissipate. When regional and open ocean resonance does occur, it can raise total global dissipation to >150% of present-day values and can cause regional macrotidal amplitudes (>2m).</p>

Bardsey – an island in a strong tidal stream: underestimating coastal tides due to unresolved topography

Bardsey Island is located at the western end of the Llŷn Peninsula in northwestern Wales. Separated from the mainland by a channel that is some 3 km wide, it is surrounded by reversing tidal streams of up to 4 m s−1 during spring tides. These local hydrodynamic details and their consequences are unresolved by satellite altimetry and are not represented in regional tidal models. Here we look at the effects of the island on the strong tidal stream in terms of the budgets for tidal energy dissipation and the formation and shedding of eddies. We show, using local observations and a satellite-altimetry-constrained product (TPXO9), that the island has a large impact on the tidal stream and that even in this latest altimetry-constrained product the derived tidal stream is under-represented due to the island not being resolved. The effect of the island leads to an underestimate of the current speed in the TPXO9 data in the channel of up to a factor of 2.5, depending on the timing in the spring–neap cycle, and the average tidal energy resource is underestimated by a factor up to 14. The observed tidal amplitudes are higher at the mainland than at the island, and there is a detectable phase lag in the tide across the island; this effect is not seen in the TPXO9 data. The underestimate of the tide in the TPXO9 data has consequences for tidal dissipation and wake effect computation and shows that local observations are key to correctly estimating tidal energetics around small-scale coastal topography.

Back to the future II: tidal evolution of four supercontinent scenarios

The Earth is currently 180 Myr into a supercontinent cycle that began with the break-up of Pangaea and which will end around 200–250 Myr (million years) in the future, as the next supercontinent forms. As the continents move around the planet they change the geometry of ocean basins, and thereby modify their resonant properties. In doing so, oceans move through tidal resonance, causing the global tides to be profoundly affected. Here, we use a dedicated and established global tidal model to simulate the evolution of tides during four future supercontinent scenarios. We show that the number of tidal resonances on Earth varies between one and five in a supercontinent cycle and that they last for no longer than 20 Myr. They occur in opening basins after about 140–180 Myr, an age equivalent to the present-day Atlantic Ocean, which is near resonance for the dominating semi-diurnal tide. They also occur when an ocean basin is closing, highlighting that within its lifetime, a large ocean basin – its history described by the Wilson cycle – may go through two resonances: one when opening and one when closing. The results further support the existence of a super-tidal cycle associated with the supercontinent cycle and gives a deep-time proxy for global tidal energetics.

Back to the Future II: Tidal evolution of four supercontinent scenarios

The Earth is currently 180 Ma into a supercontinent cycle that began with the breakup of Pangea, and will end in around 200–250 Ma (Mega-annum) in the future, as the next supercontinent forms. As the continents move around the planet, they change the geometry of ocean basins, and thereby modify their resonant properties. In doing so oceans move through tidal resonance, causing the global tides to be profoundly affected. Here, we use a dedicated and established global tidal model to simulate the evolution of tides during four future supercontinent scenarios. We show that the number of tidal resonances on Earth vary between 1 and 5 in a supercontinent cycle, and that they last for no longer than 20 Ma. They occur in opening basins after about 140–180 Ma, an age equivalent to the Present-Day Atlantic Ocean, which is near resonance for the dominating semi-diurnal tide. They also occur when an ocean basin is closing, highlighting that in its lifetime, a large ocean basin – its history described by the Wilson cycle – may go through two resonances: one when opening and one when closing. The results further support the existence of a super-tidal cycle associated with the supercontinent cycle, and gives a deep-time proxy for global tidal energetics.