深海沉積物顯示了太陽系的混沌性質，使得地質紀錄可以定年得更加準確
by Marcie Grabowski
一天是地球沿自轉軸完整旋轉一次的時間；一年是地球繞著太陽運行一圈的時間。這提醒了我們在地球上用來計算時間長短的基本單位，跟太空中地球和太陽之間的相對運動有密不可分的關係。事實上，我們絕大部分都依循著這些天文循環的節律過活。
在夏威夷外海的研究船「聯合果敢號」。圖片來源：國際海洋發現計畫(IODP)
氣侯循環也是如此。日照在一天與一年當中的循環形成了我們熟知的晝夜溫度變化與四季。在地質時間尺度下(數千年至數百萬年)，地球軌道的變化決定了冰河期的出現步調(稱作米蘭科維奇循環，Milanković
cycles)。在地質紀錄中可以看到偏心率(繞日軌道偏離完美圓形的程度，eccentricity)之類的軌道參數變化，就像指紋一樣。
天文時標(astronomical
time scale)的發展為定年地質紀錄帶來了變革，這種年代表讓我們得以運用天文學來算出地質年代的確切時間。舉例來說，地質紀錄中礦物與化學性質的周期可以配對到用天文學方法得出的周期(利用電腦逆推地球的軌道以算出過往的天文參數)。天文學方法獨自打造出一座時鐘，為地質紀錄提供了精準的年代表。
然而，地質學家和天文學家卻難以把天文時標回推到超過距今5000萬年前左右。最主要的阻礙來自於太陽系本身的混沌性質，這讓太陽系超過某個時間點之後就變得無法預測。
在發表於期刊《科學》(Science)的新研究中，夏威夷大學馬諾阿分校海洋與地球科學科技學院的海洋學教授Richard
Zeebe，以及烏特勒支大學的Lucas
Lourens提出了克服這道阻礙的方法。團隊從深海鑽探出的岩芯取得地質紀錄之後，以此作為天文學方法的限制條件，接著再用天文學方法把天文時標往前延長約800萬年。未來有望運用他們的方法把天文時標延伸至更久以前，儘管只能一個一個地質紀錄慢慢來。
另一方面，Zeebe和Lourens從南大西洋鑽出的岩芯中，分析了古新世晚期至始新世早期(約5800萬年前到5300萬年前)的沉積物數據。結果沉積物的週期性變化明顯呈現出米蘭科維奇參數的其中之一：地球軌道的偏心率。同時，Zeebe和Lourens也運算了新的天文學方法(稱作ZB18a)，結果跟南大西洋岩芯得出的數據相當一致。
「這真的令人十分震驚。」Zeebe表示，「我們從海床上鑽取超過5000萬年前的沉積物，進行分析之後得到一條曲線；接著再完全依據太陽系的物理性質和數值積分來算出另一條曲線。因此這兩條曲線是由完全獨立的方法導出，但它們看起來幾乎就像是對一模一樣的雙胞胎。」
Zeebe和Lourens並非首位發現兩者之間如此一致的科學家，但他們的突破之處在於涵蓋的時間範圍可以超過5000萬年前，其他天文學方法在此時間點之前和地質紀錄就會變得不一致。他們測試了18種發表過的方法，而ZB18a跟數據最為吻合。
他們的研究成果不只如此。利用新的年表，他們給出了古新世和始新世的新交界：5601萬年前，且誤差範圍只有0.1%。此外，他們也指出過去一個重要的氣候事件「古新世―始新世氣候最暖期」(Paleocene-Eocene
Thermal Maximum，PETM)開始的時候，偏心率也在最大值附近，代表該事件是由軌道變化引發。在類比目前與未來的人為碳排放時，科學家把PETM視為過往的最佳範例，不過PETM的引發原因仍有諸多爭議。但是現在和當時的軌道型態已經有很大的差異，代表未來軌道參數的影響可能不如5600萬年前。
Zeebe提醒：「這些軌道參數的變化皆無法直接減緩未來的暖化，因此我們還是不能輕忽人為碳排放與氣候變遷。」
對於天文學來說，這項新研究明確展現出太陽系大約5000萬年前出的混沌現象。團隊發現地球和火星軌道的頻率有所改變，進而影響了它們的振幅調變(amplitude
modulation，在音樂上通常稱作「拍頻(beat)」)。
Zeebe解釋：「你可以在幫吉他調音時聽見震幅調變的現象。當兩道音波的頻率幾乎相同，你實際上聽到的只會有一個頻率，但震幅卻會緩緩變化，這就是拍頻。」在非混沌系統中，頻率和拍頻不會隨時間變化，但在混沌系統中卻會改變(稱為共振躍遷，resonance
transition)。Zeebe繼續解釋：「拍頻的變化象徵了混沌，這讓系統變得更加有趣卻也更加複雜。矛盾的是，拍頻的變化恰好也幫我們找出新的天文學方法，進而延展天文時標。」
Deep-sea sediments reveal solar system
chaos: an advance in dating geologic archives
A day is the time for Earth to make one
complete rotation on its axis, a year is the time for Earth to make one
revolution around the Sun — reminders that basic units of time and periods on
Earth are intimately linked to our planet’s motion in space relative to the
Sun. In fact, we mostly live our lives to the rhythm of these astronomical
cycles.
The same goes for climate cycles. The cycles in daily
and annual sunlight cause the familiar diel swings in temperature and the seasons.
On geologic time scales (thousands to millions of years), variations in Earth’s
orbit are the pacemaker of the ice ages (so-called Milanković cycles). Changes
in orbital parameters include eccentricity (the deviation from a perfect
circular orbit), which can be identified in geological archives, just like a
fingerprint.
The dating of geologic archives has been
revolutionized by the development of a so-called astronomical time scale, a
“calendar” of the past providing ages of geologic periods based on astronomy.
For example, cycles in mineralogy or chemistry of geologic archives can be
matched to cycles of an astronomical solution (calculated astronomical
parameters in the past from computing the planetary orbits backward in time).
The astronomical solution has a built-in clock and so provides an accurate
chronology for the geologic record.
However, geologists and astronomers have struggled to
extend the astronomical time scale further back than about fifty million years
due to a major roadblock: solar system chaos, which makes the system
unpredictable beyond a certain point.
In a new study published in the journal Science, SOEST oceanography professor
Richard Zeebe and Lucas Lourens from Utrecht University now offer a way to
overcome the roadblock. The team used geologic records from deep-sea drill
cores to constrain the astronomical solution and, in turn, used the
astronomical solution to extend the astronomical time scale by about 8 million
years. Further application of their new method promises to reach further back
in time still, one step and geologic record at a time.
On the one hand, Zeebe and Lourens analyzed sediment
data from drill cores in the South Atlantic Ocean across the late Paleocene and
early Eocene, ca. 58-53 million years ago (Ma). The sediment cycles displayed a
remarkable expression of one particular Milanković parameter, Earth’s orbital
eccentricity. On the other hand, Zeebe and Lourens computed a new astronomical
solution (dubbed ZB18a), which showed exceptional agreement with the data from
the South Atlantic drill core.
“This was truly stunning,” Zeebe said. “We had this
one curve based on data from over 50-million-year-old sediment drilled from the
ocean floor and then the other curve entirely based on physics and numerical
integration of the solar system. So the two curves were derived entirely
independently, yet they looked almost like identical twins.”
Zeebe and Lourens are not the first to discover such
agreement — the breakthrough is that their time window is older than 50 Ma,
where astronomical solutions disagree. They tested 18 different published
solutions but ZB18a gives the best match with the data.
The implications of their work reach much further.
Using their new chronology, they provide a new age for the Paleocene-Eocene
boundary (56.01 Ma) with a small margin of error (0.1%). They also show that
the onset of a large ancient climate event, the Paleocene-Eocene Thermal
Maximum (PETM), occurred near an eccentricity maximum, which suggests an
orbital trigger for the event. The PETM is considered the best paleo-analog for
the present and future anthropogenic carbon release, yet the PETM’s trigger has
been widely debated. The orbital configurations then and now are very different
though, suggesting that impacts from orbital parameters in the future will
likely be smaller than 56 million years ago.
Zeebe cautioned, however, “None of this will directly
mitigate future warming, so there is no reason to downplay anthropogenic carbon
emissions and climate change.”
Regarding implications for astronomy, the new study
shows unmistakable fingerprints of solar system chaos around 50 Ma. The team
found a change in frequencies related to Earth’s and Mars’ orbits, affecting
their amplitude modulation (often called a “beat” in music).
“You can hear amplitude modulation when tuning a
guitar. When two notes are nearly the same, you essentially hear one frequency,
but the amplitude varies slowly — that’s a beat,” Zeebe explained. In
non-chaotic systems, the frequencies and beats are constant over time, but they
can change and switch in chaotic systems (called resonance transition). Zeebe
added, “The change in beats is a clear expression of chaos, which makes the
system fascinating but also more complex. Ironically, the change in beats is
also precisely what helps us to identify the solution and extend the
astronomical time scale.”
原始論文：Richard E.
Zeebe, Lucas J. Lourens. Solar System chaos and the Paleocene–Eocene
boundary age constrained by geology and astronomy. Science,
2019; 365 (6456): 926 DOI: 10.1126/science.aax0612
引用自：University of Hawaii at Manoa. "Deep-sea
sediments reveal solar system chaos: An advance in dating geologic
archives."
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