Waves are oscillatory motions. You can't have all the particles just leave and create a vacuum. So when the wave enters the lower impedance area there is low pressure in the high impedance area, and so particles must be drawn back in to balance it out forming a new reflected wave.

It is toxic to everything as far as I’m aware. The major danger of the bio accumulation idea is that the higher up in the food chain an organism is the more it will accumulate those toxins and die. So for example a plankton might have only a few molecules of mercury in it, but then zooplankton eats 20 of those so it has 20x the mercury in it. Then a fish eats 20 of those and so it has 400x the mercury in a plankton. And so if a human eats fish five times a week that human is potentially eating lots of mercury, and that mercury isn’t ever excreted by the body.

There’s a lot of ways we can figure out ancient sea level. There’s three main ways and they are so the sized together to try and get a complete picture. two methods both come from ocean sediment cores. Using sediment codes you can do two things. Depending on the kind of sediment you find and how old it is, you can reconstruct ancient shorelines. There are usually big sediment deposits that get left behind when sea level dramatically shifts up or down, and these are nice indicators of an ancient shoreline. Sediment cores can also be used to estimate global sea level using oxygen isotopes. Oxygen isotopes fractionate due to natural processes in the water cycle, so depending on the ratio of oxygen 18 to 16, we can take a guess at how much ice was on the land surface. Since we assume the global amount of water is somewhat constant, if we know how much is locked up on land then we can calculate how much must have filled those ocean. Combined with geological models of ancient continent shapes you can arrive at a global sea level. You can also use biostratigraphy like u/agate_ said. Corals are great for this because they live in colonies that can be tens of thousands of years old, so they allow us to see for slight changes in depth and ocean chemistry in the relatively recent past.

Also no denying historic sea level is not equivalent to a flat earther. The major reason to me is that whenever we talk about the past in science we have to put huge asterisks next to everything. We can’t ever really be certain something that happened thousands to millions of years ago happened as we think it did. However we can observe the earth as it is right now, and thus that it isn’t flat.

Fossilization is strictly the process where minerals have replaced tissue and bone, so as long as mineralization doesn’t happen you could have a bone as old as you want. It’s just hard to find them in nature because bone and tissue decays unlike minerals unless something like amber is involved.

The life cycle of a virus goes as follows. A virus attaches to their specific host, then inject their genetic material into the host. The host then gets "fooled" into reading this material and making more and more viruses. Once there's so many viruses that the host can no longer perform the function to keep itself alive, it essentially explodes and releases the next generation of viruses into the world.

Viruses can copy their DNA into their host for this purposes, but sometimes the virus is over powered or the cell survives infection and the genetic material the virus input is kept. If this happens in a gamete (sperm or egg cell) that goes on to be fertilized, that viral input is copied from the gamete to every daughter cell and voila, you have inserted viral genes into a multicellular organism.

Viruses have likely existed about as long as unicellular life, which is a very long time. And unicellular organisms have their own defenses against viruses, so once multicellular life developed the foundation of an immune system was already in place.

Fossilization is rare because of this, something like less than 1% of animals that ever lived we think made it to fossils. There's quite a few ways something can be buried quickly and avoid decomposition. Bogs and low oxygen water bodies are great at preserving as things rarely decay inside and scavengers can't get them. Mudslides and quicksand can bury and animal rapidly. In the ocean if a carcass sinks and is buried faster than microbes can break down what's left they can be preserved. Deserts are great at preserving as they are dry, so long as again the sand buries them before any scavengers majorly damage the carcass. Lots of small things fossilize much easier too. Bones of animals that have already been eaten and then excreted by an animal can survive as the poop can be fossilized. Many shell making animals are essentially pre-fossilized and so their shells can accrue in things like beaches and basins. Tree sap is also great at making fossils as amber as it can be near instantaneous.

Keep in mind that for sea creatures their bodies are made largely of water and usually have body temperatures a few degrees above ambient so they would expand as well albeit not as much.

Thank you for the correction, I had never heard of the mechanical input to the MOC. So it seems density drives the initial formation but mechanical energy is needed to have the water circulate back to surface?

The major concern is the slowing or stopping of the AMOC, the major deep water current that runs through the atlantic. Since one of the only places on earth that can make very cold and salty deep water is off the coast of greenland, and the glacier on greenland have been melting, there is much concern that this freshwater run off will cause the water near greenland to be more bouyant, causing deep water circulation to slow down or halt altogether. The consequences of this are speculative but none of them are good, as the AMOC brings important oxygen to deep water and slowing that down could cause the deep ocean to get progressively more oxygen depleted and acidic.