The following was published in the Biddeford-Saco?Journal Tribune Sunday edition, 5/24/2015.

The small bird my boys found in the backyard last weekend was olive green with an orange crown like a dirty hunters hat. It showed no signs of violence, but it was definitely dead. No rigor mortis, so it wasnt a winter casualty emerged from the snow. &thats as far as our CSI: South Portland investigation went before I got a shovel and buried the bird six inches under. My seven-year-old placed a cantaloupe-sized rock over the grave and we went on with our day.

It was only after going back inside that evening that I began to wonder what species of bird it had been.

After discussing it with a colleague at the Wells Reserve at Laudholm, I think it was either an orange-crowned warbler or a ruby-crowned kinglet. If the former, it might be a notable finding, because those warblers arent typically in Maine right now. So now Im going to dig up the carcass and send a photo to local avian expert June Ficker. My curiosity wont rest until I identify the victim. Then, at the very least, my boys can erect a more descriptive gravestone for our yards unfortunate visitor.

Why do I care? Perhaps because memorials have been on my mind lately. Earlier this month, we planted three trees at the Reserve in honor of friends and volunteers. The memory of Jeff Fletcher, a woodcutter, will live on in a sturdy red oak; Nathaniel Wright, a baseball pitcher for the Marines, Bronze Star recipient in the battle for Okinawa, and lifelong nature lover, now has a hawthorn growing in his name; and Fran Holland, part of the fabric of our early office, has an oak as well. These memorial trees should lead lives as long as the friends theyre honoring had.

Meanwhile, the Reserves sculptor-in-residence David Allen is erecting a stone entrance gate to honor our Drakes Island neighbors Ted and Kathi Exford. The Exford entrance will be the Reserves gateway to Laudholm Beach, marking Ted and Kathis lives and also our eastern boundary.

Memorials, living or stone, humble and grand, help us recall those whove left the scene. This weekend, wreaths and ceremonies will surround cenotaphs (great word) across America.

But what about memorials to entire periods in the Earths history? How do we mark the passing of time itself? Funnily enough, scientists may have recently determined the demise of a geological epoch, the Holocene, by discovering a memorial hidden in the record of the air.

A geological epoch is an amount of time in history clearly visible in layers of rock or ice laid down over time. To define an epoch, a signature that reveals a profound change to the environment must be present all over the world. For example, the Late Cretaceous epoch, when dinosaurs last cavorted through the Kennebunks, abruptly ended with a Caribbean meteor strike 65 million years ago. The distinctive dust from that meteor forms a thin, clear boundary between layers of rock dating from that time.

The Holocene epoch began 11,700 years ago with the retreat of the glaciers. Many geologists and climate scientists consider the Holocene the most recent epoch in a series stretching back 600 million years into the past. Many consider the Holocene to extend through the present day.

But a movement has been growing over the past decade to separate out a new epoch marked by human civilizations activity. The start date had been under fierce debate, but in an article in the March 12 edition of the journal Nature, geographers Mark Maslin and Simon Lewis propose that we are today living in a new human-centric epoch, the Anthropocene, which can be clearly seen by the year 1610.

Maslin and Lewis first consider some other candidates, maintaining that the proposed human epoch boundaries must be a) globally recognizable, and b) of world-changing significance. Agriculture has been around for 10,000 years, but for most of that time farmings traces have been local only. In the 1950s, nuclear weapons tests by the United States and USSR distributed a layer of radioactive dust all over the world, peaking in 1964. Fortunately for us, thats all those weapons have done  they havent irreversibly changed the environment, so they cant be counted as an epoch marker either.

Which brings us back to the year 1610. Starting in 1492 with Christopher Columbuss discovery of the New World, the Eurasian and American continents became biologically connected. Columbus and those explorers and traders who came after him brought tomatoes, corn, tobacco, cocoa and more back to Europe. Pollen from those New World plants begins to appear in the fossil record of the Old World after 1492, due to trade and human activity. But that was still a local effect  not yet a strong enough signal on which to base a new epoch.

Colonial trade didnt just go one way, of course. Eurasian peoples introduced grapes, livestock, sugarcane, and other foreign species to the Americas (they also inadvertently brought earthworms to New England). The crux is that they also brought diseases: smallpox, typhus, measles. Within a half dozen generations of 1492, more than 50 million native North and South Americans had died from contagions. With their deaths, their farms expired too. Native American agriculture quickly reverted to forest and jungle, and all those new trees sucked up carbon dioxide from the atmosphere.

There is a worldwide dip in CO2 recorded in air bubbles trapped in Antarctic ice. Because the atmosphere is global, that dip, centered on the year 1610, is the best candidate for the end of one epoch and the beginning of another unlike any other. Clearer than fallout dust or local industrial ash or pollen grains, theres a memorial in the air trapped in ice 405 years ago. The last known major decrease in carbon dioxide is not just a memorial to 50 million humans; it is a gravestone on the Holocene, the last epoch on earth untouched by human hands.

Nik Charov is president of Laudholm Trust, the nonprofit partner of the Wells National Estuarine Research Reserve in Wells, Maine. His Sunday column, Between Two Worlds, ventures forth from the intersection of art and science, past and future, life and death. More at wellsreserve.org/twoworlds.